Lie, which is the more popular definition;
or a period of 20 minutes.
“No cap” refers to no lie being told.
“Andrew, let’s meet up in one cap.”
“Okay Justin, I’ll see you in twenty.”
—> A cap is a twenty minute time period, so these people are planning on meeting up in twenty minutes.
“Devon, your playlist is really good no cap.”
—> Someone is telling Devon that his playlist is honestly very good.