Oh, not just the Yiddish, et', but the etymology trail wwh posted and this specifically:

lith TUKTI TO BECOME FAT
TAUKAS ANIMAL FAT sug> LARD


The word 'taukas' [animal fat] just sounded so much like
the 'tucchus' you have taught me to spell correctly, 'et. And so I thought perhaps that whever tucchas came into being in whatever spelling it first came to refer to the butt, I think perhaps it could have referred to the fatness of the tucchus that led to its moniker. Just a probably too obvious observation.