Green’s Dictionary of Slang

1. a person who shoots, a killer.

1890		J.P. Quinn Fools of Fortune 404: When a raid is made [...] enough ‘pluggers’ are captured to fill one or two wagons and are driven to the nearest police station.
1919		E. Dyson ‘Billjim’ in ‘Hello, Soldier!’ 52: Down to it is Plugger Bill, / Lyin’ crumpled, white ’n’ still.
1939		Howsley Argot: Dict. of Und. Sl.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US) a male copulator.

1893

‘Bob Sterling’ Town-Bull 11: ‘Ah, my lusty plugger. I wil [sic] see if I cannot get that pretty pieceof virtue to take a taste of this [i.e. a penis].