crust is a multipurpose word which has no true definition. it has been used to describe any kind of flaky skin caused by a multitude of conditions eg leprosy or excema. it is also used in moments of anger or just in casual speech. very prominent in the city of london school for boys.
"look at that crust on mr maynard's elbows, look at his crusty hands"
"lets crust around for a while" (lets mess around)
"this work is such a pile of crust"
"what the crust are you on about?"
"arghh man what the crust is this?"