A piece of sea wit, sported in hot weather. I am becalmed, the sail sticks to the mast; that is, my shirt sticks to my back. His prad is becalmed; his horse knocked up.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
beau trap * beckNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes