What's the etymology of 'chupar'?
***Excellent question. According to DRAE and to Portuguese sites, it's onomatopoeic. I suspect it's some sort of substrate or possibly from another Ibero-Romance language -perhaps with /s-/ > /ch-/ for intensive purposes and perhaps /-k-/ > /-p-/ if we're looking at a NE origin. OTOH, Portuguese <ch> /sh/ is cognate to Spanish <ll> and is from Latin <cl-, fl-, pl->.
So, if looking at it from the NE, I'd say, look at sopa "soup" > chupa (and there is a soup called chupe), if looking from the NW, then pulpar > plupar > chupar.
I'm sure someone will come up with a Northern Caucasus etymology