1
general::
Cheer up, become encouraged, as in Buck up! We'll soon have it done, or Even the promise of a vacation did not buck her up. This term was first recorded in 1844.
American Heritage Idioms
2
general::
Phrase(s): buck up
to cheer up; to perk up. • Come on, now, buck up. Things can’t be all that bad. • She began to buck up when I showed her the results of the tests.
McGrawhill's American Idioms And Phrasal Verbs