Signifyin' is the roots of rap - hangin on the corner with your buddies, giving each other a hard time; "I heard about yo mama . . .".

It has another name in blues - the Dozens. "Your mama and your cousins are dirty by the dozens . . .". A typical example was quoted somewhere above - with the line "dirty mother for ya don't ya know" [which is, in plain text, motherf****r]. Jimmy Johnson, Elmore James' piano player, recorded several great versions of this, but it dates back to the 20's and 30's as well.

Both of the quotations you asked about are from the blues tradition of explicit - but thinly veiled - sexual references.