The common Proto-Germanic prefix *ga‑ affixed to past participles was reduced in Modern English, obscuring its historical participial morphology now beyond modern recognition, as seen for example, in:
- a‧like from Old English anlic, reinforced by Old Norse glikr.
- hand‧i‧work from Old English handġeweorc < hand + ġeweorc.
- a‧mong from Middle English ymong and this from Old English onġemang from on “in” + ġemang “mingling”.
- The regional prefix a‑ as in a-been, a-scattered, a-muddled, a-ready (which can still be found in southwest England and the southern United States).
From these examples, it seems that a simple vowel sound remained in the initial, unstressed position after any initial consonant sound before it had been worn down. We know that there had originally been one from comparison within the Germanic language family.
For several reasons that shouldn’t matter here, I still associate this prefix with palatal consonantal sounds in English, such as in:
Also Francophone gently /ˈd͡ʒɛntli/.
The diphthongization seen in the Modern English first-person singular nominative pronoun I /aj/ with /j/ (sometimes represented as /aɪ/, and corresponding to Latin ego), that is consonantal in regional cham, "I am".
Does this prefix surface anywhere in a form that would ever sound like ja‑ as in English jar /d͡ʒɑr/, or at least like ya‑ as in English yahoo /ˈjɑhu/?
In this case there may be some minute chance that jam-packed could just possibly be akin to German gepackt, the past participle of packen meaning “pack” and used in virtually the same sense in German as it is in English, although often with a leading intensifier: voll, völlig ∼ full, fully.