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1.
A person who only shows up to an event or occasion every so often and not all the time.

Its name derives from people who only go to church on Sunday and not on other days on the church calendar.
Guy 1 - Where's John? We haven't hung out with him in ages.
Guy 2 - He's just a Sunday Boy, forget about him.
by JohnSmithLad August 05, 2011