Green’s Dictionary of Slang

(orig. US) a general excl. of excitement, pleasure, surprise, amazement.

1848		D.O. Madden Revelations of Ireland 115: Whew, boy! what a passion old Biddy is in.
1894		Ade Stories of the Street and of the Town (1941) 24: S-s-t! Boy! Same as the last time.
1911		G. Stratton-Porter Harvester 552: I just stood in the carriage and screamed, ‘Boy! Boy!’.
1918		J.E. Rendinell diary 10 July in One Man’s War (1928) 134: Boy! A perfect day.
1922		S. Lewis Babbitt (1974) 50: ‘How do you feel, the morning after the night before?’ ‘Oh, boy! Some head!’.
1933		R. Chandler ‘Blackmailers Don’t Shoot’ in Red Wind (1946) 103: Mallory looked at Costello, and said in a strained, angry voice: ‘Boy, are you lucky!’.
1935		C.G. Finney Circus of Dr Lao 75: Boyoboy [...] I wonder what word the Greeks had for that!
1947		N. Marsh Final Curtain (1958) 219: ‘Gee,’ she said, ‘will that sound good! Boy, oh boy!’.
1953		J. Thompson Alcoholics (1993) 26: Boy, if I don’t get a drink fast I’m going to fall apart.
1956		I. Fleming Diamonds Are Forever (1958) 160: It’s twenty Grand for a rub these days, Boy-oh-boy!
1961		V.S. Naipaul A House For Mr Biswas 449: Boy! You had me frightened.
1972		C.E. Palmer Wooing of Beppo Tate 116: He scooted. Boy, did he scoot!
1978		A. Maupin Tales of the City (1984) 182: Boy! [...] Brian showed me what the landlady grows in the garden.
1985		B. Humphries Traveller’s Tool 124: Boy oh boy, does that word ‘homework’ take me back.
1998		P. McCabe Breakfast on Pluto 16: Boy, Braden! What a place!
2000		C. Cook Robbers (2001) 57: Della looked down at her bloodred hands. Boy oh boy.
2000		Indep. Traveller 8 Jan. 7: And boy, is a rainforest rainy.
2007–8		Mail and Guardian (S. Afr.) 7 Dec.–3 Jan. 53: Dilettantes from the city [...] will tell you the Karoo is flat and boring. Boy, are they mistaken.