Literally, one who frequents the barbershop for frequent haircuts. This is early Tudor Slang -- the Historical Thesaurus to the OED gives 1605 as the only year of use for this -- and the only reason it appears in unabridged dictionaries at all is that it's used in King Lear. Possibly it's a Shakespeare coinage that never took off? Regardless, his usage makes the best example -- from act II, scene ii, as Kent confronts Oswald:
Draw, you whoreson cullionly barber-monger, draw!