early 15c., "soft material for padding or stuffing," of uncertain origin, and the different meanings may represent more than one source. Among the possible connections are Medieval Latin wadda, Dutch watten, and Middle English wadmal (late 14c.) "woolen cloth," which seems to be from Old Norse vaðmal "a woolen fabric of Scandinavia," probably from vað "cloth" + mal "measure."
The meaning "bundle of currency" is American English, 1778. To shoot (one's) wad "do all one can do" is recorded from 1914. The immediate source of the expression probably is the sense of "disk of cloth used to hold powder and shot in place in a gun." Wad in slang sense of "a load of semen" is attested from 1920s, and the expression now often is felt in this sense. As a suffix, -wad in 1980s joined -bag, -ball, -head in combinations meaning "disgusting or unpleasant person."
1570s, from wad (n.). Related: Wadded; wadding.
[1914+; fr the wad of cloth formerly stuffed into a gun barrel to keep the shot and powder in place]