"small fore and aft rigged vessel with one mast, generally carrying a jib, fore-stay sail, mainsail, and gaff-topsail," 1620s, from Dutch sloep "a sloop;" probably from French chaloupe, from Old French chalupe "small, sloop-rigged vessel," which is perhaps related to English shallop [OED]. But according to Barnhart and Watkins the Dutch word might simply be from Middle Dutch slupen "to glide," from PIE *sleubh- (see sleeve). In old military use, a small ship of war carrying guns on the upper deck only (1670s).