mid-15c., "fortified place, stronghold," from Old French fort "fort, fortress; strong man," noun use of adjective meaning "strong, stout, sturdy; hard, severe, difficult; hard to understand; dreadful, terrible; fortified" (10c.), from Latin fortis "strong, mighty; firm, steadfast; brave, spirited," from Old Latin forctus, possibly from PIE root *bhergh- (2) "high, elevated," with derivatives referring to hills and hill-forts (see barrow, n.2). Or possibly from *dher- (2) "to hold firmly, support." Figurative use of hold the fort attested from 1590s.