(US, idiomatic, slang, euphemistic) In secret.
I'll tell you, but keep it on the down-low.
(US, idiomatic, slang, euphemistic) Secretly sleeping with someone other than one's partner.
(African-American Vernacular, LGBT) (of a male) Publicly identifying as heterosexual but secretly having sex with other men.
Source: en.wiktionary.orgon the down low