He's got more -- than Carter's got pills.
Hotter'n a two-dollar pistol
Hotter than Kelsie's nuts
Dry as a popcorn fart
(Something) went over like a fart in a space suit (or diving bell)
Slicker than a newborn weasel (or snot on a doorknob)

The problem with these things is that they are low-hanging fruit for
free association - they just keep on comin'.

And a really old western (Colorado?) expression, "Well, don't that take the rag off the bush, though?" The best explanation I've heard for that one relates to crude trail markers left by scouts for those following - a piece of cloth (rag)tied to the branch of a shrub or limb of chaparral. If someone wanted to screw things up royally, they had only to "take the rag off the bush."