"a revolving machine consisting of wooden horses or seats mounted on a circular platform," 1729, from merry (adj.) + go-round. Figurative use by 1838. Merry-totter (mid-15c.) was a Middle English name for a swing or see-saw. Also compare merry-go-down "strong ale" (c. 1500); merry-go-sorry "a mix of joy and sorrow" (1590s).