Green’s Dictionary of Slang

nails! excl.

an oath.

c.1386		Chaucer Doctor-Pardoner Link 2 (Text C) line 288: Our Hoste gan to swere as he were wood, ‘Harrow!’ quod he, ‘by nayles and by blood!’.
[1535		R. Copland Hye Way to the Spyttel House line 360: There they reuell as vnthryfty braggers, With horyble othes swerynge as they were wood, Armes, nayles, woundes] .
1573		New Custom II iii: His nails, I would plague them one way or another .
1600		Dekker Shoemakers’ Holiday III i: Fooles? nailes! if I tarry nowe, I would my guts might be turnd to shoe-thread.
1604		Dekker Honest Whore Pt 1 V ii: Nailes, I think so, for thou telst me.