guard

v. & n.
—v.
1 tr. (often foll. by from, against) watch over and defend or protect from harm.
2 tr. keep watch by (a door etc.) so as to control entry or exit.
3 tr. supervise (prisoners etc.) and prevent from escaping.
4 tr. provide (machinery) with a protective device.
5 tr. keep (thoughts or speech) in check.
6 tr. provide with safeguards.
7 intr. (foll. by against) take precautions.
8 tr. (in various games) protect (a piece, card, etc.) with set moves.
—n.
1 a state of vigilance or watchfulness.
2 a person who protects or keeps watch.
3 a body of soldiers etc. serving to protect a place or person; an escort.
4 US a prison warder.
5 a part of an army detached for some purpose (advance guard).
6 (in pl.) (usu. Guards) any of various bodies of troops nominally employed to guard a monarch.
7 a thing that protects or defends.
8 (often in comb.) a device fitted to a machine, vehicle, weapon, etc., to prevent injury or accident to the user (fire-guard).
9 Brit. an official who rides with and is in general charge of a train.
10 in some sports: a a protective or defensive player. b a defensive posture or motion.
Phrases and idioms:
be on (or keep or stand) guard (of a sentry etc.) keep watch. guard cell Bot. either of a pair of cells surrounding the stomata in plants. guard-rail a rail, e.g. a handrail, fitted as a support or to prevent an accident. guard ring Electronics a ring-shaped electrode used to limit the extent of an electric field, esp. in a capacitor. guard's van Brit. a coach or compartment occupied by a guard. lower one's guard reduce vigilance against attack. off (or off one's) guard unprepared for some surprise or difficulty. on (or on one's) guard prepared for all contingencies; vigilant. raise one's guard become vigilant against attack.
Derivatives:
guarder n. guardless adj.
Etymology: ME f. OF garde, garder ult. f. WG, rel. to WARD n.