1570s, "comedic or mirthful entertainment," from from obsolete verb merry "be happy; make happy" (Old English myrgan; see merry (adj.)) + -ment. General sense of "state of being merry, mirth" is from 1580s.
mermaid
merman
mero-
Merovingian
merrily
merriment
merrow
merry
merry man
merry widow
merry-andrew