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I don't know how 'worthy' came to be pronounced with a /ði/ whereas 'healthy', 'wealthy', 'stealthy' etc., are pronounced with a /θi/. I am aware that theories of pronunciation in English are far too complex and often seem illogical. But with a curious student in my front wanting to know why 'worthy' has the /ði/, my task becomes very difficult.
I would appreciate if you could let me in on something that might help.
This is a fascinating question, and I have no idea how that particular voicing came to be. Roger Lass is probably who I'd ask about that.
I was interested enough, though, to look at the words in Webster's 2nd ending in -lthy and -rthy.
( egrep rthy$ webster2.txt, etc. )
They turn out to have a very interesting pattern.
In each group (which are roughly the same size, in terms of number of root words), there is, first, a smattering of individual words like swarthy and filthy, including healthy and unhealthy, stealthy and unstealthy.
And then there comes a flurry of compound words made from a single root word; both of these words refer to the same concept, though in two flavors:
As can be seen, words ending in -worthy are vastly more diversified and specialized, to the extent that -worthy has become something of a libfix, like -ectomy. This may or may not have anything to do with the voicing; I wouldn't know.
The Data
. . . R . . . . . .. L . .
birthy . . . . . filthy
earthy . . . . healthy ~ unhealthy
forthy . . . . stealthy ~ unstealthy
swarthy
airworthy . . . . . overwealthy
Allworthy . . . . . subwealthy
blameworthy . . superwealthy
bloodworthy . . . ultrawealthy
bribeworthy . . . unstealthy
evenworthy . . . unwealthy
faithworthy . . . wealthy
fameworthy
hangworthy
helpworthy
honorworthy
keepworthy
laughworthy
loveworthy
markworthy
mootworthy
newsworthy
noteworthy
oathworthy
painsworthy
praiseworthy
preworthy
prizeworthy
quoteworthy
respectworthy
roadworthy
seaworthy
shameworthy
showworthy
sightworthy
songworthy
stageworthy
talkworthy
thankworthy
thegnworthy
trustworthy
unnoteworthy
unpraiseworthy
unroadworthy
unseaworthy
untrustworthy
unworthy
viewworthy
wonderworthy
worshipworthy
worthy
In the case of /'wɛlθi/ vs. /'wɚði/, we're looking at more than one hypothetical rule, and the more I look into it, the more it screams ?exception. Of course, much like OP says, English phonetic rules are kind of made to be broken and/or drive ESL learners insane.
Judging from the excellent data set collected by @John, I believe it is a combination of two phonetic rules, one based on manner of [l_] vs. [ɹ_], and one based on voicing of the fricatives when followed by /i/ (I won't go into other vowels, but there is a pattern).
First rule: left over from OE, unvoiced fricatives to voiced when followed by a (high) vowel.
Second rule: R-vocalization and r-colored vowels influence voicing in /'wɚði/. The influence of /l/ on the interdental fricative, however, is not sufficient to ease into a comfortable voiced pronunciation.
The slight difference in /l/v and /ɹ_/v -- liquid vs. rhotic -- may be enough to trump the First rule based on the influence of the rhotic /ɹ_/ on the fricative. Therefore we see a voiceless "th" when preceded by /l/.
That there are so few -orthy words (c.f. all the words that can add "-worthy" to make an adjective), but with the limited data set and with the very relevant OED citation from @Alex with "worthy" making an appearance long before "-lthy," later to be followed by "wealthy..." @John, I think I see where you're going....
Both /l/ and /r/ are alveolar and located very close in the mouth, but with the slight difference in pronunciation when it comes to approximates. Interesting to note that in languages like Japanese, which does not differentiate these phonemes, many native speakers do not notice the difference between the pronunciation of /l/ and /ɹ/ in English.
Could this be the case of the Great Vowel Shift?
Since the meanings of the two words are so closely related, I checked the etymology of wealth:
From Middle English welth, welthe, weolthe ("happiness, prosperity"), alteration (due to similar words in -th: compare helth ("health"), derth ("dearth")) of wele ("wealth, well-being, weal"), from Old English wela ("wealth, prosperity"), from Proto-Germanic *welô (“well-being, prosperity”), from Proto-Indo-European *wel- (“good, best”), equivalent to weal + -th. Cognate with Dutch weelde ("wealth"), Low German weelde ("wealth"), Old High German welida, welitha ("wealth") Related also to German Wohl ("welfare, well-being, weal"), Danish vel ("weal, welfare"), Swedish väl ("well-being, weal"). from here
and worth:
From worth or wurth, from Old English weorþ, from Proto-Germanic werþaz (“towards, opposite”) (the noun developing from the adjective). Cognate with German wert/Wert, Dutch waard ("adjective"), Swedish värd.
Keep in mind that "þ" (thorn) evolved into "th." One might just toss the potential rules at this point.
Always handy: IPA chart
ðis a voiced dental fricative andθis a voiceless dental fricative. – donothingsuccessfully Jan 30 '13 at 19:06ðis the beginning of "this" whileθbegins "thin" – simchona Jan 30 '13 at 19:56