Green’s Dictionary of Slang

(US campus) to a slight / limited extent.

2001		(con. 1990s) in J. Miller One of the Guys 176: ‘“Oh, girl, they just be jokin’ around. We just be doin’ it on,” they call it “the low-key”’.
2010		Urban Dict. 26 Feb. 🌐 I didn’t think I would be but I’m lowkey sad that my roommate is moving out.
2014		C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 Fall 4: LOWKEY — with slight inclination: ‘I lowkey want food right now’.
2016		C. Eble (ed.) UNC-CH Campus Sl. Spring 2016 6: LOW KEY — slightly, subtly, discretely: ‘I lowkey told my roommate to pick up after herself’ ; ‘I lowkey wanted to present first, but I didn’t say anything’ Semi-secretly.