Although not fact checked, I think it was a rejunvination of the MacDaddy name to introduce a new model of wedge much like the "big Bertha" was for the drivers.
Heres where I cant confirm. It must have been the name of a good wedge back in the day.
The MD line has done very well so the name is sticking,,,,,, so far.
Hey guys... found this in an old PGA Tour post. Apparently, Phil coined the name Mack Daddy. Here is an excerpt from the article. "According to Cleveland, Mickelson hit a full lob wedge shot about 40 yards and watched as the ball sucked back about 40 feet. The four-time major winner looked at the ball, then down at the face of the wedge, and blurted out: "That's my Mack Daddy groove.""