Guinea, n.² — Green’s Dictionary of Slang

Guinea n.²

[the common use of words with the initial G in this way, e.g. gosh n., golly n.¹]

(US) a euph. for God, e.g. swear to Guinea; also as v.

1922		G.B. McCutcheon Viola Gwynn 306: Why, I’ve been livin’ with that woman for seventeen years and I swear to Guinea she’s still got me puzzled.
1938		J.H. Warner ‘Word-List from Southern Arkansas’ in AS XIII:1 6/1: Guinea [...] This is an expression of the ‘Old South’ according to incomplete evidence. ‘I guinea! That horse can run.’.