abbreviation for down down low, which is an extreme form
of the rather common dl
or down low
to the necessity of keeping a juicy tidbit
of information hushed up.
"Whoa, you hooked up with HIM?!?!?!?!"
"Yeah, but let's keep it on the ddl. He's pretty skeevy. I was too drunk to know what I was doing."
"Oh, okay, I won't say a word."