Green’s Dictionary of Slang

1. a London milkman [his supposed watering down of milk with chalky water].

1865		Daily Tel. 7 Sept. (?) n.p.: It is an ominous fact that London milkmen are known in the vocabulary of slang as chalkers [F&H].
1866		Adelaide Obs. (SA) 17 Mar. 6/8: [from Notes & Queries, London] Chalker, London Slang fob Milkman [...] This expressive synonym f [...] has never, I believe, yet found its way into any slang dictionary.

2. (US) a bookmaker [the chalking up of odds].

1907

News and Courier (Charleston, SC) 14 Apr. 18/1: They’d hop after one of these cruising chalkers to make mind bets.

3. (US campus) an extremely obese person [to measure their girth one has to reach as far round as poss., make a chalk mark, and repeat round the other side to finish the circle].

1990

Eble Campus Sl. Oct. 1: chalker – person who is too large in girth to be encircled in one’s arms.