Is it ensure, insure, or assure?
1570s, in poetry, from French couplet (mid-14c.), a diminutive of couple (see couple (n.)). In music, from 1876.
A pair of lines of verse that rhyme. Some poems, such as “The Night Before Christmas,” are written entirely in couplets:
`Twas the night before Christmas, when all through the houseNot a creature was stirring, not even a mouse; The stockings were hung by the chimney with care In hopes that St. Nicholas soon would be there.