Green’s Dictionary of Slang

grab n.²

In exclamations

by grab! (also by grabs!)

(US) a euph. for by God!

1887		Scribner’s Mag. n.p.: Dadgum ye! [...] by grabs, hit’s a human critter! [F&H].
1902		Laurens Advertiser (SC) 15 Jan. 4/1: If you will, by grab, you can lick me if I don’t come to church.
1908		‘O. Henry’ ‘Roses, Ruses and Romance’ in Voice of the City (1915) 139: ‘By grabs!’ he exclaimed.
1912		Van Loan ‘Ten-Thousand-Dollar Arm’ in Ten-Thousand-Dollar Arm 42: ‘Why don’t you raise Steve’s salary?’ ‘By grab! [...] I will!’.
1929		J. White ‘The Old Chisolm Trail’ in Lonesome Cowboy 7: It looked, by grab, like we was goin’ to lose ’em all.
1930		S. Henry Conquering Our Great Amer. Plains 74: The wind began to blow an’ rain began to fall; An’ it looked, by grab, like we was goin’ ter lose ’em all [DA].
1959		N. Nye Long Run (1983) 216: ‘By grab,’ the deputy was saying, ‘you have sure saved the county—’.