early 13c., "a seat" (as in Siege Perilous, the vacant seat at Arthur's Round Table, to be occupied safely only by the knight destined to find the Holy Grail, c.1230), from O.Fr. sege "seat, throne," from V.L. *sedicum "seat," from L. sedere "sit" (see sedentary). The military sense is attested from c.1300; the notion is of an army "sitting down" before a fortress.