Maybe everybody’s heard of this but me, but I was only introduced to the term being on the down low recently. Like so much of our slang, it originates from African-American culture:
Men who discreetly have sex with other men while in sexual relationships with women are said to be on the “down low” (or “dl” for short). Often these men do not consider themselves gay or bisexual and their female partners are not aware that they have sex with other men, thus the term “down low” or “in hiding”.
There’s even a book about it.
UPDATE: Bah. I thought of this crack too late, but I suppose if you’re not on the down low, you’re on the up-and-up. Hmm. In retrospect, that was hardly worth adding.