Green’s Dictionary of Slang

slap-happy adj.

[SE slap + -happy sfx; orig. boxing use, someone whose brain has suffered from an excess of fighting]

cheery, slightly eccentric.

[US]J. Tully Bruiser 107: A slap-happy bum.
[US]Mezzrow & Wolfe Really the Blues 76: He was taking potshots at the electric bulbs, with a slap-happy grin on his kisser.
[UK]Hastings & St Leonards Obs. 14 July 6/4: They were a little ‘sla-happy about book-keeping’.
[US](con. 1920s) ‘Harry Grey’ Hoods (1953) 131: They were ten-dollar bill slap-happy. They went into every store in the neighborhood trying out your samples.
[US]D. Pearce Cool Hand Luke (1967) 39: The same horseplay and slaphappy jokes of fresh conscripts going into the army.
[UK]W. Trevor Angels at the Ritz 187: She’d been a slap-happy mother and was a slap-happy wife, not caring much what people thought if her child wasn’t as meticulously turned out as other children.
[US]L. Heinemann Paco’s Story (1987) 161: Slaphappy people look stupid, loony — and drivers think you’re on something.
[US]Arizona Republic (Phoenix, AZ) 17 Jan. 54/1: Sla-happy ‘Ninja’ needs less dialogue.
[US](con. 1975–6) E. Little Steel Toes 62: Couple of slaphappy motherfuckers, giggling like little girls.
Sth Florida Sun Sentinel (Fort Lauderdale, FL) 29 Jan. G11/3: Book Review. Slap-happy spy caper takes aim at Florida.
[US]Maryland Indep. (Port Tobacco, MD) 15 Feb. B16/5: Maybe it was the slap-happy 10 pm clicking around on my smart-phone but I was briefly tempted.