gladiolus n.

mid-15c., "Roman swordsman," from Latin gladiator (fem. gladiatrix) "fighter in the public games; swordsman," from gladius "sword" (there is no verb *gladiare), which probably is from Gaulish (compare Welsh cleddyf, Cornish clethe, Breton kleze "sword;" see claymore). Old Irish claideb is from Welsh.

The close connection with Celtic words for 'sword', together with the imperfect match of initial consonants, and the semantic field of weaponry, suggests that Latin borrowed a form *gladio- or *kladio- (a hypothetical variant of attested British Celtic *kladimo- 'sword') from [Proto-Celtic] or from a third language. [de Vaan]

"coarse grass-like plant growing in wet places," Middle English segge, from Old English secg "sedge, reed, rush," according to Watkins from Proto-Germanic *sagjaz "plant with a cutting edge" (source also of Low German segge, German Segge), from suffixed form of PIE root *sek- "to cut," on notion of plant with "cutting" leaves.

Compare Old English secg, identical in form but meaning "sword;" and German schwertel-gras "sedge" from schwert "sword," also see the etymological sense of gladiolus). Old Irish seisg, Welsh hesgreed "rush" might represent a similar sense development from the same root. Often spelled seg, segg until the present form triumphed early 1900s. Related: Sedgy (early 14c., seggy).