Bolt1  —n. 1 sliding bar and socket used to fasten a door etc. 2 large metal pin with a thread, usu. Used with a nut, to hold things together. 3 discharge of lightning. 4 act of bolting. —v. 1 fasten with a bolt. 2 (foll. By in, out) keep (a person etc.) In or out by bolting a door. 3 fasten together with bolts. 4 a dash off suddenly. B (of a horse) suddenly gallop out of control. 5 gulp down (food) unchewed. 6 (of a plant) run to seed. —adv. (usu. In bolt upright) rigidly, stiffly.  bolt from the blue complete surprise. [old english]