serious answer
@tauon (I'll say at first that I am just a nerd for this stuff, but not very smart, so I might get stuff wrong, if someone notices innacuracies, pls correct me)It actually used to mean something along "to succeed", like asking "How are you making out?" was something like "Are you doing okay?"
Then, because of that meaning, it started to get used more as a slang of something like "succeeded in having sex", and since tongue kissing was associated with sex, it got attached to that meaning.