waist pack vs butt pack

waist pack and butt pack both are nouns.

Word	Noun	Adjective	Verb	Adverb
waist pack	Yes	No	No	No
butt pack	Yes	No	No	No

As nouns, butt pack is a hyponym of waist pack; that is, butt pack is a word with a more specific, narrower meaning than waist pack:

waist pack: a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist
butt pack: a waist pack worn with the pouch in back

Other hyponyms of waist pack include fanny pack.

waist pack (noun)	butt pack (noun)
a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist	a waist pack worn with the pouch in back