Green’s Dictionary of Slang

(US Und.) the very lowest class of prostitute.

1859		Wkly Varieties (Boston, MA) 29 Oct. 7/2: We wonder where Van Pelt, faro-bank pimp and snkeak [...] got his new clothes? Guess his ‘lofter’ must have ‘bilked’ a countryman.
1866		Night Side of N.Y. 95: Diseased and filthy, their faces bloated and haggard [...] they have reached that point where further debasement is impossible [...] These women, so lost, so fallen, are called ‘lofters’ in the police vocabulary.