That sounds like an urban legend
to me. Although it is cited several places around the web, I've also heard that the word is onomatopoeic
- The war officers would check to see if the bladder was inflated by flick
ing the sack. If it responded with a *blimp
* sound, it was at the proper pressure.
Wordorigins.org (an all around great website, btw) also points out that J.R.R. Tolkien claims the word is a combinations of the words blister and lump, which sounds plausible enough. Not much more so than the b-limp theory, though. I like the *blimp* theory, myself.