Green’s Dictionary of Slang

1. cheer up!

1892		F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 42: ‘Buck up,’ he sez.
1898		W. Pett Ridge Mord Em’ly 75: ‘Buck up!’ said Mord Em’ly to herself.
1907		Gem 30 Mar. 10: ‘Oh, buck up!’ said Manners. ‘Don’t be a milksop!’.
1914		‘Bartimeus’ ‘The Greater Love’ in Naval Occasions 216: Buck up, Shortie! [...] it’s Saturday night at Sea! Your night for a glass of port.
1920		F.S. Fitzgerald This Side of Paradise in Bodley Head Scott Fitzgerald III (1960) 103: Oh, Amory, buck up!
1924		L. O’Neil ‘Mate o’ Mine’ in Dinkum Aussie and Other Poems 4: Your hand on my shoulder whenever it sagged— ‘Buck up!—and I’d straightaway buck.
1930		N. Lindsay Redheap (1965) 105: ‘Buck up, J.B.,’ said Robert. ‘You presume to encourage me,’ said Mr Bandparts, becoming sardonic.
1934		H. Miller Tropic of Cancer (1963) 197: Come on, buck up! Don’t let him think you’re croaking.
1947		(con. 1944) J.H. Burns Gallery (1948) 243: Buck up, Guilia, said an officer [...] Life is real, life is earnest.
1951		Cusack & James Come in Spinner (1960) 246: Buck up, dear, time enough to cry when your turn comes.
1966		T. Keyes All Night Stand 159: Come on Faro, buck up.
1973		A. Bennett Habeus Corpus Act I: Come on, buck up.

2. hurry up!

1899		Boy’s Own Paper 21 Jan. 261: Go it, my kiddies; buck up.
1908		Gem 17 Oct. 7: Buck up, dear boy. It’s awfully bad form to keep a chap waitin’.
1914		C. Holme Lonely Plough (1931) 261: Buck up, I tell you!
1922		‘J.H. Ross’ Mint (1955) 19: Buck up, old seat-wiper: I can’t tip you and I’m urgent.
1937		R. Westerby Wide Boys Never Work (1938) 169: Those damned Bankleys will be here in a minute, Maggie [...] Buck up, for Gawd’s sake.
1958		A. Buckeridge Jennings’ Little Hut 47: Buck up, Darbi, it’s your turn.
1997		(con. 1950s) J. Byrne Slab Boys [film script] 58: Buck up, son ... thur’s been quite a few in here we huvnae seen.

3. come on! improve yourself! put more effort in!

1905		Sporting Times 18 Mar. 10/2: Buck up ye school boards.
1927		(con. WWI) C. Venable ‘An Argonne Raid’ in Mason Fighting American (1945) 474: Nonsense! Buck up!
1938		F. Anthony ‘Rivals’ in Me And Gus (1977) 64: He faltered and then hissed in my ear, ‘Buck up! You look like a scarecrow’.
1949		R. Park Poor Man’s Orange 267: Buck up and get hold of the pieces and pull them together.