A beard grown not for an aesthetic purpose but merely because the person wearing it has not gotten around to shaving.
A: Growing a beard for winter?
B: Nah, it's just a lazy beard...I'll shave it once I figure out where I put my razor.
When someone lets his beard grow because he is too lazy to shave it he grows lazy beard.
Woman: "Why are you growing beard? It doesn't look good at all."
Man: "Yeah I know but it's lazy beard."
Woman: "Oh, then it's okay!"