early 15c., "one who keeps watch, a body of soldiers," also "care, custody, guardianship," and the name of a part of a piece of armor, from French garde "guardian, warden, keeper; watching, keeping, custody," from Old French garder "to keep, maintain, preserve, protect" (see guard (v.)). Abstract or collective sense of "a keeping, a custody" (as in bodyguard) also is from early 15c. Sword-play and fisticuffs sense is from 1590s; hence to be on guard (1640s) or off (one's) guard (1680s). As a football position, from 1889. Guard-rail attested from 1860, originally on railroad tracks and running beside the rail on the outside; the guide-rail running between the rails.
mid-15c., from guard (n.) or from Old French garder "to keep watch over, guard, protect, maintain, preserve" (corresponding to Old North French warder, see gu-), from Frankish *wardon, from Proto-Germanic *wardon "to guard" (from PIE root *wer- (3) "perceive, watch out for"). Italian guardare, Spanish guardar also are from Germanic. Related: Guarded; guarding.
"one who guards," early 14c., garden; early 15c., gardein, from Anglo-French gardein (late 13c.), Old French gardien "keeper, custodian," earlier guarden, from Frankish *warding-, from the Germanic source of guard (v.). Specific legal sense is from 1510s. Guardian angel is from 1630s.
late 14c., sauf-gard, "protection, security, defense," from Old French sauve garde "safekeeping, safeguard" (13c.), from salve, sauve (fem. of sauf; see safe (adj.)) + garde "a keeping" (see guard (n.)). Meaning "one who protects, something that offers security from danger" is recorded from late 15c.
1530s, "scullion, kitchen knave," of uncertain origin. Perhaps in reference to military units or attendants so called for the color of their dress or their character; more likely originally a mock-military reference to scullions and kitchen-knaves of noble households, of black-liveried personal guards, and of shoeblacks. See black (adj.) + guard (n.). By 1736, sense had emerged of "one of the idle criminal class; man of coarse and offensive manners." Hence the adjectival use (1784), "of low or worthless character."