pisser vs bed wetter

pisser and bed wetter both are nouns.

Word	Noun	Adjective	Verb	Adverb
pisser	Yes	No	No	No
bed wetter	Yes	No	No	No

As nouns, bed wetter is a hyponym of pisser; that is, bed wetter is a word with a more specific, narrower meaning than pisser:

pisser: a person who urinates
bed wetter: someone suffering from enuresis; someone who urinates while asleep in bed

Other hyponyms of pisser include bedwetter, wetter.

pisser (noun)	bed wetter (noun)
a very disagreeable difficulty	someone suffering from enuresis; someone who urinates while asleep in bed
a person who urinates