Dissimilation: 'taper' from 'papyrus'

Question

I was researching the etymology of taper {verb} which motivated this question. Observe that Etymonline's entry for the verb just rechannels to that for the noun:

taper (n.)
Old English tapur, taper "candle, lamp-wick," not found outside English, possibly a dissimilated borrowing from Latin papyrus (see papyrus), which was used in Medieval Latin and some Romance languages for "wick of a candle" (such as Italian papijo "wick"), because these often were made from the pith of papyrus.

I don't know the meaning of dissimilated; so I researched it:

dissimilate {verb} {with object} {Linguistics} = 1. Change (a sound or sounds in a word) to another when the word originally had identical sounds near each other (e.g. in taper, which derives from papyrus, the p is dissimilated to t).

Alas, this definition looks circular. 1. Would someone please clarify which p in 'papyrus' ODO meant to target? Please disambiguate the bolded p?

2. Would someone please enlarge on the 'dissimilation' here? What's so special about the pronunciation of 'papyrus'? How did it feature identical sounds near each other ?

Answer 1

Assuming that "taper" derives historically from "paper", word-initial t can be said to have dissimilated, because it originally was p, which is totally similar to the second p in the word, and now it is less similar (has a different place of articulation). Dissimilation is, in fact, not limited to totally-identical segments, it also includes segments that are identical with respect to some feature. For instance, in /b...g/, the segments b and g are identical with respect to voicing, but not place of articulation; in /p....b/ the segments are identical with respect to place of articulation but not voicing. This word does not reflect a systematic sound change, so we can't say anything about the causal mechanism behind this transformation. In Akkadian and some Berber languages, such dissimilation (labial to lingual before labial) is more systematic, but this is a nonce change.

Answer 2

Here’s what Lyle Campbell in Historical Linguistics (an introductory textbook) has to say about dissimilation:

Dissimilation, the opposite of assimilation, is change in which sounds become less similar to one another. Assimilation is far more common than dissimilation; assimilation is usually regular, general throughout the language, though sometimes it can be sporadic. Dissimilation is much rarer and is usually not regular (is sporadic), though dissimilation can be regular. Dissimilation often happens at a distance (is non-adjacent), though contact dissimilation is not uncommon. (2nd ed, p. 28)

An example of assimilation is Latin octo > Italian otto ‘eight’. In this case the change is a contact one (c is next to t) and is also regular (Latin ct will change to tt in most environments in Italian.)

By contrast, the paper > taper dissimilation is non-adjacent (a vowel intervenes) and irregular, which according to Campbell is typical of the process. It’s interesting that English retains ‘paper’ as a general term, ‘taper’ having a more narrow semantic range.