Something that you beat people with.
Worse than a cat-o'-nine-tails.
The most evil, hideous punishment imaginable.
A limp bit of starchy stuff about 12" long.
For some reason, people (usually those over fifty or otherwise old-fashioned) would threaten their kids with "I'm going to beat you with a wet noodle if you don't do X!", with the understanding that said beating would involve something much more painful than a simple noodle. Also used by elementary school teachers to threaten their students, but without the actual threat of corporal punishment.
My grandfather (he's 85) uses this figure of speech still. Maybe he has a jar of Super-Jumbo Spaghetti Noodles, three feet long and an inch in diameter, soaking in a bath of pure capascin so that they'd burn when they struck you...