A derogatory slur suggesting that ones preferred medium of expertise is the male genitalia. In the same way the stonemason works in stone, the cockmason works in cock.
Guy 1: Don't be such a cockmason.
Guy 2: Wow, that's a really clever insult, you got me there.
Guy 1: Why thank you, I appreciate the compliment, sorry for calling you a cockmason.
Guy 2: It's ok, you wanna go to the bar?
Guy 1: No.