"wishbone of a fowl's breast," c. 1600, from merry (adj.) + thought. So called from the sport of breaking it between two persons pulling each on an end to determine who will get a wish he made for the occasion (the winner getting the longer fragment). Also see wishbone.
merry man
merry widow
merry-andrew
merry-go-round
merrymaking
merrythought
Mersey
mesa
mesalliance
mescal
mescaline