Green’s Dictionary of Slang

(orig. US black gay) a form of emphatic terminal interjection, ‘and that’s that!’, found on line.

2019		Urban Dict. 26 Apr. 🌐 When someone explains that they are done with something or someone at the end of their sentence. ‘I know you ain’t talking with yo dusty, rusty, musty, crusty ass! Periodt!’.
2020		dictionary.com Periodt comes from period, used as an interjection to show a statement is final, that there is nothing else to be said or debated.
2024		https://www.today.com 23 Feb. 🌐 [headline] How did words like periodt, GYAT, cap and drip come to be? All about the Black history of slang .