I must say, I thought it was "short shift," as in the manager doesn't like you and schedules you, say, 30 hours a week instead of the full 40. Kind of similar to the actual meaning of "curt dismissal."
Please don't laugh.
So, the archaic definition of "shrift" was (according to my awesome dictionary widget- bless you, Apple!) the penance imposed by a priest after confession. The archaic expression of "short shrift" meant a short time between a judge's condemnation and punishment.
There you have it!