1. (orig. US) the least appetizing, poorest, most run-down and poss. dangerous area of a city or town; often as armpit of the nation/universe; also in phr. rough as a docker’s armpit.
2. (orig. US) used similarly of a place, e.g. a nightclub or café.
3. (US campus) an unpleasant individual.
[mid-19C] (UK Und.) to cheat an accomplice out of his share of the proceeds.
SE in slang uses
[19C+] (orig. US) consumed by, overwhelmed by.
[early 19C] (UK Und.) to confine one’s criminality to such activities that would be classed as petty larceny, bringing a maximum sentence of seven years transportation, rather than hanging; thus work above the armpits v., to commit crimes that could lead to one’s execution.