Green’s Dictionary of Slang

grin n.3

(US) amusement; thus for (shits and) grins, for fun.

in IUFA Folk Speech n.p.: It could be grins: something could be fun [HDAS].
[UK]J. Sullivan ‘Thicker than Water’ Only Fools and Horses [TV script] karen: Your Dad’s a giggle ain’t he? del: Yes Karen, that man is one long grin!
[US](con. 1968) D.A. Dye Citadel (1989) 67: Maybe take Steve along for grins.
[US]B. Hamper Rivethead (1992) 72: This new requirement was a real grin.
[US]J. Lansdale Rumble Tumble 107: Maybe I should hit him some more, just for grins.
[UK]K. Sampson Powder 86: They’re having a grin.
[US]G. Pelecanos Night Gardener 218: So, for shits and grins, you’re saying we should compare the markings.
[US](con. 1962) J. Ellroy Enchanters 83: ‘I buy cherry sno-cones [...] douse them with vodka and green Chartreuse. It’s kicks and grins, baby’.

In phrases

ain’t life a grin

(US campus) a cynical expression of misfortune.

[US]Eble Campus Sl. Mar. 1: Ain’t life a grin? – expression used when everything is going wrong.
on a grin (adv.)

(US) without difficulty, ‘laughing’.

[US]Gleaner (Manchester, NH) 29 July n.p.: He can beat Davy Crockett all hollow on a grin.
on the grin (adj.)

smiling, cheerful.

[US]S. Ford Torchy 248: I was on the grin all the afternoon too, thinkin’ of the joshes I was goin’ to hand him.