Upside down over idioms

Just a thought: Does it bother anyone else that to be “up for” and “down for” something mean the same thing?

From a couple of quick Google searches, it seems the former is more established as a proper English idiom (though, to be fair, I am defining “proper” as not having Urban Dictionary too high up on the search results). Then the question becomes, when did we decide to lower ourselves?

I imagine an angsty teenager somewhere, some time ago, saying to herself, “Ugh, life is so hard. I’m not up for anything. From now on, I’m down to do something.”

Unlikely.

Maybe it was another teenager in rebellion with all of his buddies, ditching class to smoke behind the local preschool: “Hey, you — you know. You down for that cool secret thing we’re going to do?”

“Yeah, I’m down, man. I’m down.”

I wonder if there are implicit connotations that dictate which idiom to use in certain contexts. Would I be up only for a challenge, a dare, an adventure? Is there an implication that I have to rise to some occasion? Conversely, would I be down only for guilty pleasures or things beneath me? Down for a marathon of Katherine Heigl romantic comedies? Down for transferring to Stanford? (Obligatory “GO BEARS!” It is Big Game Week, after all.)

Or, as a homeless guy once said to me as I passed him on the street, DTF? It occurs to me that might be where the latter idiomatic usage originated. But if that’s the case, homeless sir, wouldn’t it make more sense to be UTF?

Either way, no. I am not down nor up to fondle.

Damn idioms.