bunk vs lower berth

bunk and lower berth both are nouns.

Word	Noun	Adjective	Verb	Adverb
bunk	Yes	No	Yes	No
lower berth	Yes	No	No	No

As nouns, lower berth is a hyponym of bunk; that is, lower berth is a word with a more specific, narrower meaning than bunk:

Other hyponyms of bunk include lower, upper, upper berth, couchette.

bunk (noun)	lower berth (noun)
a bed on a ship or train; usually in tiers	the lower of two berths
a rough bed (as at a campsite)
beds built one above the other
a long trough for feeding cattle
unacceptable behavior (especially ludicrously false statements)
a message that seems to convey no meaning

bunk (verb)	lower berth (verb)
flee; take to one's heels; cut and run
provide with a bunk
avoid paying