#2: “oligosynthesis” > “litchsomdeed”
Ancient Greek *ὀλιγοσύνθεσις < *h₃ligo-som-dʰeh₁-tis > PGmc *lika-sam-dēdiz > OE liċsamdēd > ModE litchsomdeed /ˈlɪtʃsɔmdiːd/
(Note: while ὀλιγοσύνθεσις is not attested I will act like it was, and so on for future posts.)
PIE form: ὀλίγος is one of those words that's really just inherently internally reconstructible (it's ambiguous whether it's g or ǵ but that doesn't matter), which is good because the only other cognate is Armenian. σύν is harder, and I've chosen to reconstruct *sóm per Beekes (if I balk every time I see a root that we don’t know the reconstruction of, I’m going to be doing precious few posts.) θε-σις is the much-clearer *dʰeh₁-tis, which also has an attested Germanic descendant in...
Germanic form: *dēdiz, which is kind of surprising, since because the PIE word is barytone you'd expect *dēþiz (and if it were from an oblique form you’d expect *dadiz.) But because attested irregularities trump regular sound changes in my book, I go with *dēdiz (if it were *þ you’d have “litchsomdeeth”.) Back to the first part of the compound: this too is pretty elementary; the initial laryngeal disappears, *g > *k by Grimm's law. (As for *dʰeh₁tis, *eh₁ > *ē, *dʰ > *d and *t > *þ by Grimm, *þ > *d and *s > *z by Verner.) *som becomes *sam.
Old English form: *k is palatalized by the preceding *i. *a is syncopated out (good riddance). *sam stays the same (it *could* become som, and if this word were real those forms would probably both be attested, becoming “litchsomdeed”) but *iz > *i in West Germanic, and the *i is lost due to high-vowel loss.
Modern English form: Final ċ is spelled “tch” after a short vowel. ē becomes ME /e:/, spelt <ee>, and this spelling survives into English (besides, it's really just "deed" anyway.)
It’s also arguable that *som would assimilate into *son the way it did in Greek and Latin, both of which liked prepositions a *lot* more than English does. However, it’d be without precedent, and it’s worth noting that German does not assimilate anything (e.g. anbringen).
