Green’s Dictionary of Slang

1. an affirmative excl. absolutely! genuinely!

1961		R. Gover One Hundred Dollar Misunderstanding 165: I tell her that, she gonna pee her pants. Fer real!
1972		B. Rodgers Queens’ Vernacular 84: for real [serial, sure, true] [...] 3. an affirmative. “Ques: ‘You’re tired of having to pay for it, right?’ Ans: ‘For true!’”.
1985		T.R. Houser Central Sl. 23: for real [...] ‘Dude got a gun, for real!’.
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 For real (fo’ real) (interjection) 1. That’s true; I agree.
2002		Teen Lingo: The Source for Youth Ministry 🌐 fo’ real derived from ‘for real.’ Meaning yes, that is very true. ‘Mrs. B. give us so much #$%^ homework!’ ‘Fo’real!’.
2018		‘Aidan Truhen’ Price You Pay 175: Yeah okay you gotta say this guy belongs in post man. Like for realz.

2. as a negative excl., implying alarm or threat.

1997–2000

College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 For real (fo’ real) (interjection) [...] 2. Statement of alarm. 3. Statement of threat. [Hawaii] (primarily used by athletes).