I took a quick gander at that scene in episode 42, and it seems to me like he's doing the "air" version of a common slapping/smacking motion that is prevalent in manzai, a form of Japanese comedy. Manzai is performed by two comedians, the dumb one (boke) and the smart one (tsukkomi). The boke will say and do stupid/disagreeable things, and the tsukkomi will make retorts, often violently. This is where the variety of different slaps and smacks come in. If the boke says something stupid or something that the tsukkomi takes offense to, the tsukkomi will slap them along with a verbal retort. What Usopp is doing here greatly resembles that, but I'm not 100% sure.