fall in vs buckle

fall in is not a noun while buckle is a noun.

Word	Noun	Adjective	Verb	Adverb
fall in	No	No	Yes	No
buckle	Yes	No	Yes	No

As verbs, buckle is a hyponym of fall in; that is, buckle is a word with a more specific, narrower meaning than fall in:

Other hyponyms of fall in include go off, implode, crumple, flop, break, sink, slide down, slump.

fall in (noun)	buckle (noun)
	fastener that fastens together two ends of a belt or strap; often has loose prong
	a shape distorted by twisting or folding

fall in (verb)	buckle (verb)
break down, literally or metaphorically	bend out of shape, as under pressure or from heat
become part of; become a member of a group or organization	fasten with a buckle or buckles
to take one's place in a military formation or line	fold or collapse