plug, n.² — Green’s Dictionary of Slang

plug n.²

[? lit. or fig. plays on SE plug, i.e. it ‘fills a gap’; sense 1 abbr. plughole]

1. the mouth.

1715

‘Nickydemus Ninnyhammer’ Homer in a nut-shell 18: With that he took a potent Jug, / And strait advanc’d it to his Plug.

2. of alcohol.

(a) a draught of beer.

1816

‘Quiz’ Grand Master VII 184: Come, Sir, another plug of malt [OED].

(b) wine, esp. cheap wine.

1970

3. a translation, a ‘crib’ [? it ‘plugs up’ the gaps in one’s knowledge].

1854		‘Cuthbert Bede’ Adventures of Mr Verdant Green (1982) II 238: Getting up his subjects by the aid of those royal roads to knowledge, variously known as cribs, crams, plugs, abstracts, analyses, or epitomes.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 49: plug, n. Literal translation.

4. (US) a silver dollar [ext. use of SE plug, a small piece of solid material used to stop up a hole].

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 49: plug, n. A silver dollar.
1902		‘The Wasp’ Tales of the Penance Track’ in Bulletin 31 May 31/2: Rajah Riley, having won a small pool the week before, plunged on Revenue to the extent of ten plugs (or ‘long tailors’).