Someone who is doing something they are not supposed to be doing. A sneaky, shady, and/or sinful person. The word developed in Harlem, NY as a replacement swear word to use around kids.
"Quit bein' a merv and get off the block to lead a decent life."
"Uncle Joe was mervin' out the other day. I caught him s-t-r-i-p-p-i-n-g at the bar."