Green’s Dictionary of Slang

cheer up! don’t worry!

1936		Auden & Isherwood Ascent of F6 II iii: Chin up! / Kiss me! / Atta Boy! / Dance till dawn among the ruins of a burning Troy!
1947		R. Goffin Horn of Plenty 255: Let’s go, Zutty—chin up!
1951		D. Stivens Jimmy Brockett 149: Chin up, sister.
1960		G. Swarthout Where the Boys Are 20: Finally I said chins up.
1975		T. Rhone School’s Out II iii: Chin up, man, chin up.
2005		Entrepreneur.com 🌐 [headline] Chin Up! Overcome any challenge through the power of positive thinking.
2023		R. Milward Man-Eating Typewriter 185: ‘Chin up, old chap’.