early 13c., "a seat" (as in Siege Perilous, early 13c., the vacant seat at Arthur's Round Table, according to prophecy to be occupied safely only by the knight destined to find the Holy Grail), from Old French sege "seat, throne," from Vulgar Latin *sedicum "seat," from Latin sedere "sit" (see sedentary). The military sense is attested from c.1300; the notion is of an army "sitting down" before a fortress.