Items that are used as examples of something limp:
1) A noodle. (cooked noodles are flexible and droopy)
2) Wet washcloth. (they can be crisp or stiff when dry, but go shapeless in water)
3) Dish rag / rag. (the generic form, also handkerchief, especially if wet as above)
4) Wet leaf. (as above)
5) Hair. (hair that has no body, especially fine, straight, or blonde hairs)
*Bizkit. (the name of the rock band Limp Bizkit is a phallic reference)
This isn't an idiom. When you see AS___AS___you're dealing with A Simile. This simile is comparing something to a pair of old, patched socks, which would indeed be very limp.
simile its a simile when you use "like"
ANSWER:drowned kitten
Historically, the simile has been "deaf as a door post". *NOTE: Today, these kinds of comparisons would be seen as rude.
fiddle
mule
As dear as salt
Niagra Falls morning.
...mule."
as blue as sea
my teacher was as surprised as a dog
A simile. A simile compares two unlike items and specifically uses like or as in the comparison.