Green’s Dictionary of Slang

1. (US black) a state of secrecy.

2002

(con. 1998–2000) J. Lerner You Got Nothing Coming 99: Tells me to ‘keep it on the D.L.’ ‘What’s the D.L., Skell?’ [...] ‘The D.L., dawg, is the down-low. You keep something on the D.L. and it stays with you — you down wid that?’.

2. (US black) ostensibly heterosexual, married men who enjoy sex with homosexual men.

2005

N.Y. Blade 27 May 🌐 While no one tried to dismiss the reality of the down low, everyone on the panel wondered why the ‘DL’ eclipsed all other issues and concerns when the conversation turned to sexuality, increasing HIV rates, and the black community.