"pressed close together," 1667 (in "Paradise Lost"), probably a past participle adjective from serry "to press close together" (1580s), a military term, from Middle French serre "close, compact" (12c.), past participle of serrer "press close, fasten," from Vulgar Latin *serrare "to bolt, lock up," from Latin serare, from sera "a bolt, bar, cross-bar," perhaps from PIE *ser- (3) "to line up" (see series). Modern use is due to the popularity of Scott, who used it with phalanx.