"planking or woodwork round the uppermost parts of a vessel," early 15c., from Middle Dutch bulwerke or Middle High German bolwerc, probably from bole "plank, tree trunk" (from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell;" see bole) + werc "work" (see work, n.). Figurative sense "means of defense or security" is from 1570s.