The u in guard is doing a job that has nothing to do with pronunciation: it keeps the g hard. French inserted u after g before a front vowel for exactly this reason, and English inherited the convention in guard, guest, guide, guile and guilt. Where you see gu- at the start of an English word, French spelling practice is usually the reason.
The word itself is Germanic in origin, from a root meaning to watch or protect, which passed into Frankish and then French, where the w became gu-. English took it back in the early 1400s from French garde. So guard is a Germanic word that went to France, acquired a French spelling convention, and returned — which is why it sits beside the native-descended ward as a doublet. The fencing and boxing senses date from the 1590s, giving on guard and off one's guard.
guard (n.) 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. also from early 15c. guard (v.) 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 . also from mid-15c.
guard came in through Norman French, and its spelling follows that family's conventions.
How that family writes these sounds
| hard /g/ before e or i | gu-, with a u wedged in to keep the g hardguard, guest, guide, guile |
What gives it away on the page
| gu- | A u inserted to keep the g hard before a front vowel.guard, guest, guide, guile |
Norman French in the spelling guide · guess another word's family