flagger, n. — Green’s Dictionary of Slang

flagger n.

[SE flag, a paving stone, upon which she walks; + ? her showing the flag/flag-about under flag n.⁵]

a street prostitute.

1865

Daily Paper Police Report n.p.: She wasn’t a low sort at all she wasnt a flagger, as we call it. So I replies, I am well, thankee; and am happy to say I feel as such [F&H].