guard (n.) Look up guard at Dictionary.com
early 15c., "one who keeps watch," from Middle 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.
guard (v.) Look up guard at Dictionary.com
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" (see ward (v.)). Italian guardare, Spanish guardar also are from Germanic. Related: Guarded; guarding.