What does Cranly mean by a "sugar"?

A "sugar' is a former friend.

A dull scowl appeared on Cranly’s forehead. He stared at the table where Moynihan had bent to write his name on the roll, and then said flatly:

—A sugar! —

Quis est in malo humore, said Stephen, ego aut vos?

Cranly did not take up the taunt. He brooded sourly on his judgement and repeated with the same flat force:

—A flaming bloody sugar, that’s what he is!

It was his epitaph for all dead friendships....