Definition of "cheer up"

cheer up

verb
third-person singular simple present cheers up, present participle cheering up, simple past and past participle cheered up
1. (intransitive, idiomatic) To become happier or less downcast.
2. (transitive, idiomatic) To make someone happier or less downcast.
  Quotations
  Therefore cheere vp your mindes, prepare to fight,He that can take or ſlaughter Tamburlaine,Shall rule the Prouince of Albania.
  c. 1587–1588, [Christopher Marlowe], Tamburlaine the Great. […] The First Part […], 2nd edition, part 1, London: […] [R. Robinson for] Richard Iones, […], published 1592; reprinted as Tamburlaine the Great (A Scolar Press Facsimile), Menston, Yorkshire, London: Scolar Press, 1973, (please specify the page)
interjection
1. An encouragement to somebody who seems unhappy.