Green’s Dictionary of Slang

(US) that is that, there is no more to be said.

[1701		Cibber Love Makes a Man I i: If it were not for my hopes in young Clody, I might fairly conclude my Name were at a Period].
[1713		W. King York Spy 33: He’s the Creditor’s Hawk [...] the Period of young Gentlemen, or their Full stop for when he meets ’em they can go no further].
1934		J.H. O’Hara Appointment in Samarra (1935) 248: ‘An unscrupulous woman can make a man—’ ‘Period.’.
1947		R.O. Boyer Dark Ship 159: These guys are no good union busting finks—period.
1954		J. Thompson Swell-Looking Babe 125: She’d got it period.
1965		H.S. Thompson letter 5 Feb. in Proud Highway (1997) 491: McGarr got pretty jittery about me using his phone; pretty jittery period.
1978		D. DeLillo Running Dog (1992) 186: ‘Keep it, period,’ she said.
1986		C. Hiaasen Tourist Season (1987) 89: That’s the last chapter of the Sparky Harper case. Period.
1992		T. Williams Crackhouse 63: The mission is going to get drugs. Period. Whether it’s in the street, in a private spot, or whatever.
2001		N. Barlay Hooky Gear 107: That, as you would put it, is bollocks. Period. End of.