Green’s Dictionary of Slang

a sarcastic excl. implying that one is shedding (utterly unfelt) tears at a given comment or event.

1864		Kentish Chron. 27 Feb. 6/6: Boo-hoo! what will he do / When the Greek tax-collector walks into Corfu?
1888		Springfield Dly Republican (OH) 9 Jan. 4/4: They stick their fist in the eyes and bawl Boohoo! Boohoo! Boohoo! You babies!
1902		St Louis Republican (MO) 6 July 48/2: We want some jam too! Boo-hoo! Boo-hoo!
1935		J. Conroy World to Win 61: Terry recited this with mock solemnity, and the more playful boes punctuated it with ‘boo-hoo’ and wiping their eyes.
1942		P. Larkin letter 21/28 July in Thwaite Sel. Letters (1992) 39: Oh boo hoo. You lucky fellow to be in the Army away from it all.