Short for "you know what I'm sayin'." Pronounced "ya kweese." Can be lengthened for effect. "ya kwEEEEEEESE." Not an internet acronym, but a truncated
verbal form of "you know what I'm sayin'."
Correct response to 'ykwis' is not 'ikwis (I know what ya sayin') but another 'ykwis.'