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Webster 1913 Edition

Noun.

One who, or that which, pushes.

Noun.

One that drives forward.

pusher ‎(plural pushers)

Someone or something that pushes. [from 16th c.]
(military slang) A girl or woman. [from 20th c.]
- 1929, Frederic Manning, The Middle Parts of Fortune, Vintage 2014, p. 208:
  ‘You should a seed some o' the pushers. Girls o' seventeen painted worse nor any Gerties I'd ever knowed.’
(colloquial) An illegal drug dealer. [from 20th c.]
(aeronautics) An aircraft with the propeller behind the fuselage. [from 20th c.]

that which pushes

drug dealer

aircraft