Searching for a clue, I see that Julian Sanchez was fishing around the other day for an answer. He doesn't seem to get anywhere but he provokes one commenter to remind us of what George Orwell wrote about "dying metaphors" in "Politics and the English Language":
DYING METAPHORS. A newly invented metaphor assists thought by evoking a visual image, while on the other hand a metaphor which is technically ‘dead’ (e. g. iron resolution) has in effect reverted to being an ordinary word and can generally be used without loss of vividness. But in between these two classes there is a huge dump of worn-out metaphors which have lost all evocative power and are merely used because they save people the trouble of inventing phrases for themselves.I've deeply internalized Orwell's advice. (I've read the old essay many times.) When I was writing the first sentence of this post, for "I always pick some other phrase," my first thought was "I always hold my tongue," and I rejected that phrase
Yet I do think "in the tank" sound spiffy. I like it. But I'm never going to use it until I know what the tank is.
ADDED: Some people seem to think I don't understand what "in the tank" means, and many think they've solved my problem by saying that "tanking" is a boxing term connected to "taking a dive." But what I want is a concrete image for the "tank" that we are talking about. I understand that a boxer taking a fall very eagerly can be pictured as "diving" the way you would dive into a pool or a tank of some sort. So if we say someone is "in the tank" as opposed to "tanking," do we mean that he has so eagerly gone for someone that he's not only taking a dive, he has already dived, so he's "in the tank"? In that case, it seems to be a sort of large clear-sided aquarium of the sort a circus might use for a stunt or a magician might vow to remain submerged in for a week. So if you think someone is "in the tank for Obama," picture a tank like that.
SO: Is The Tank in the tank?