There seems to be only a few words starting with the 'gw' sound, though the only ones I can think of are proper nouns, for example, Gwyneth. Are there any non-name words starting with a 'gw'? Is there any relationship between a real 'gw' and a 'gu' pronounced as 'gw' (say, iguana)?
In the particular cases you mention, they're not related, though they potentially could be, sort of. I'll try and explain:
- the /gw/ of "Gwyneth" (and a few other names) comes from Welsh, which appears to have an alternation between /w/ and /gw/ at the start of words (so "Winnie" and "Gwinnie" are essentially variants of the same name); such variation occurs in other languages including Spanish, so e.g. "huevo" and "guevo" ("egg") are variants of the same word;
- the /gw/ of "iguana" is the result of a process in various Romance languages whereby a high vowel is automatically diphthongised with a following vowel-- this process occurs fairly automatically in French and Spanish. To to a Spanish speaker, the /gw/ of "iguana" isn't a special combination as such-- it's just the result of an automatic process that occurs any time "u" and "a" occur one before the other.
By these two routes, the combination has accidentally entered English in these words. However, arguably the processes are related: arguably a contributing factor to the [g] sound in the first case is diphthongisation of the [u] vowel.
There are quite a few words beginning with the /gw/ sound, spelled with gu-. (Neil Coffey’s answer excellently explains why the two different spellings exist.) To name just a few well-known ones:
- guano (and many derivatives: guanine, etc.)
- guiro (a percussion instrument)
- guar (guar gum is a common stabilising ingredient in foods)
And of course many demonyms:
(Replying to the first question.)
Yes, there are some words which are non-names but many are obsolete. From the OED (I didn't write them all):
- gwacum obs. form of guaiacum.
- gwairdoun obs. Sc. form of guerdon.
- gwakin see #1
- gwan, g'wan U.S. and Irish Dial. pronunciation of "go on"
- gwane, gwayn obs. ff. guana, the Iguana.
- gwarri var. guarri.
- gwash Anglicized form of gouache.
- gwely - [Welsh.] A. A social unit that was once traditional in Wales, consisting of four generations of one family in which the great-grandfather, the head of the group, had proprietary right over its landed property. B. The land held by the members of such a group.
- Gwentian - n. and a. |ˈgwɛntɪən| [f. the name Gwent + -ian.] A. Noun a. An inhabitant of Gwent in Monmouthshire, historically a Welsh principality. b. The dialect of this region. B. adjective - Of or pertaining to Gwent.
- gwerddoun, gwerdon see #2
- gwerre - variant of guerre, obsolete.
- gwidege - var. guides, obsolete, the jugular vein.
- gwis - obs. Sc. form of goose.
Don't forget this one:
guano |ˈgwänō| noun ( pl. -nos) the excrement of seabirds, occurring in thick deposits notably on the islands off Peru and Chile, and used as fertilizer. [NOAD]
guacamole |ˌgwäkəˈmōlē| noun a dish of mashed avocado mixed with chopped onion, tomatoes, chili peppers, and seasoning.
Not many words starting with [gw] exist in English.
I think the two languages that were identified in the previous comments and answers, Welsh and Spanish, are the main sources of words starting with /gw/ in English. It may also be worth mentioning that many Spanish words starting with [gw] are not "native" Spanish vocabulary, but ultimately from other languages such as Arabic, Nahuatl or Quechua. (There are apparently some dialectal sound changes in Spanish that can yield word-initial /gw/ in native words—e.g. Wiktionary tells me that güey originated as a variant form of buey—but I don't know of any English word starting with gu- that comes from a source like this sound change.)
Native English and Latin words don't start with [gw]
Words starting with [gw] do not exist in inherited native English vocabulary. Latin is a major source of non-inherited English vocabulary, but [gw] did not occur at the start of native Latin words either. In the middle of certain Latin words, such as lingua "tongue, language" and extinguo "extinguish", there was something like [gw]: it was written as <gu>, and is sometimes analyzed phonemically as a single consonant phoneme /gʷ/. My understanding is that it could only be found after the sound [ŋ] (which in this context was written as <n>, and is typically analyzed as an allophone of the phoneme /n/).
English, Latin and the other Indo-European languages are hypothesized to have a common ancestor, which we reconstruct as "Proto-Indo-European" ("PIE" for short). PIE did have sounds conventionally transcribed as *gʷ and *gʷʰ, but neither of these yielded word-initial [gw] in either English or Latin. Furthermore, there are no other sounds or sequences of sounds in PIE that regularly yielded word-initial [gw] in either English or Latin. So the absence of word-initial [gw] in the native vocabulary of English and Latin can be "explained" (in a way) by these facts about PIE phonology and the sound changes between PIE and English and between PIE and Latin.
This isn't a complete "explanation" because it doesn't tell us anything about why the sound changes were what they were.
Spanish words/names starting with gu- that originally came from other languages
A number of words starting with [gw] exist in Spanish; many are from non-native sources like Arabic or various American languages (e.g. Nahuatl and Quechua). In Spanish, <gu> is the regular way of writing [gw] (as well as similar, more lenited sounds like [ɣw]—Spanish has allophonic variation between voiced velar plosives, which are phonetically transcribed as [g], and voiced velar fricatives or approximants, which are phonetically transcribed as [ɣ]).
As Neil Coffey said, the glide [w] can be analyzed as a non-syllabic allophone of the vowel phoneme /u/ in Spanish, although I don't think the analysis of glides as non-phonemic in Spanish is entirely uncontroversial (see "Quasi-Phonemic Contrasts in Spanish" (José Ignacio Hualde, 2004) for some discussion). Regardless of the correct synchronic analysis of [gw] in Spanish phonology, I haven't yet found any example of a Spanish word where [gw] is etymologically derived from a sequence of /g/ + a historically syllabic vowel /u/.
Historically, Spanish has often used [gw] to adapt words that start with the glide [w] in other languages. E.g. the first part of Guadalupe, Guadalajara, and Guadalcanal is supposed to be from Arabic wadi. Guano is from Quechua wanu.
Romance gu- from Germanic [w]
Germanic [w] was taken into Romance languages (Spanish and also French and Italian) as gu-, much as in the examples in the previous section.
However, I can't actually think of any English words that start with [gw] because of this. The words guarantee and guard are from Germanic roots with [w] that passed through French to enter English, but the original [w] ended up being lost.
The Italian name Guido apparently has given rise to a derogatory slang word "guido", so maybe that would count.
Welsh words/names starting with gw-
As mentioned in other posts, word-initial [gw] exists in Welsh, including in inherited Welsh words. Unlike Spanish, Welsh makes frequent use of the letter W in writing: it is used to represent the glide [w] as well as the syllabic vowels [ʊ] and [uː]. The letter U is used in Welsh orthography to represent a different vowel sound.
Welsh is the source of names like Gwendolen, Gwenda, Gwen, Gwyneth, as well as the fish-name gwyniad mentioned in Wayfaring Stranger's answer.
Other sources of word-initial [gw]
There are some other sources of word-initial [gw] that seem to be so infrequent that it wouldn't make sense to have an entire section devoted to each one. So here are some words of "miscellaneous" origins:
gouache (or "gwash", as in Alenanno's answer): from French gouache, which the OED says is from Italian guazzo (apparently French both borrowed and phonetically adapted the word), which Wiktionary says is "probably from a variant of acquazzone". So in this word, the [gw] seems to be from Latin -qu- in medial position
guar: from Hindi (the sources I've looked at give different spellings and transliterations of the Hindi word, so I'm not sure about its exact form, but it seems to start with /g/ and have /w~v/ somewhere later in the word)