As it says in the title, I'm hoping to find out where the adjective ending -っぱい comes from (like in 酸っぱい 'sour'). Is it related to -っぽい (X-like, X-ish) at all? I can't seem to find an etymology for it if there is one known, but I really only have access to free resources like wiktionary and anything I can find on a google search and isn't behind a paywall, and that hasn't brought up anything.
From what I can find, the ～っぱい ending is peculiar to two words: しょっぱい and すっぱい.
Shogakukan's 国語大辞典 and Daijirin both suggest that the ～っぱい ending for しょっぱい is a shift from 映【は】ゆし ("blindingly bright"; as an auxiliary element, "to appear XX; to be conspicuously XX"). Digging around, I also found Shogakakun(https://kotobank.jp/word/%E9%B9%B9%E6%98%A0%E3%82%86%E3%81%84-516930#E3.83.87.E3.82.B8.E3.82.BF.E3.83.AB.E5.A4.A7.E8.BE.9E.E6.B3.89), Daijirin, and Daijisen entries for 鹹【しおはゆ】い, which appear to corroborate this derivation.
Separately, I suspect that the more-common ～っぽい suffix may also derive from this same 映【は】ゆし, although I cannot currently find any Japanese source that says anything more than simply that っぽい is from ぽい, and that ぽい is a suffix essentially meaning -ish, -like.