n. An alum who graduated a significantly long time ago. Crust begins accreting on a graduate at the moment of Commencement. The longer a person has been out of Williams, the crustier he or she is.
adj. Having crust. An alum is technically "crusty" when all the classes he or she overlapped with at Williams have graduated. (ie: after Commencement 2005 the '02's became officially crusty because the last class they were at Williams with, the '05's, graduated).
More informal definitions of crustiness include when you don't have a floor to crash on anymore, and when you don't have someone to guest meal you anymore.
The term is used primarily in the Odd Quad, to apply to Odd Quadders (note that even Crusties are still Odd Quadders. Once an Odd Quadder, always an Odd Quadder).