early 12c., "a fortunate day," also, generally, "good fortune;" from good (adj.) + day (n.). As a salutation in parting, haue godne day "have good day" is recorded from c. 1200; good day as a greeting is from late 14c.
good den
Good Friday
good morning
good will
good-bye
good-day
good-for-nothing
good-humored
good-looking
goodly
goodman