light arm vs weapon

light arm and weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
light arm	Yes	No	No	No
weapon	Yes	No	No	No

As nouns, weapon is a hypernym of light arm; that is, weapon is a word with a broader meaning than light arm:

Other hypernyms of light arm include arm, weapon system.

light arm (noun)	weapon (noun)
a rifle or pistol	any instrument or instrumentality used in fighting or hunting
	a means of persuading or arguing