Fais do-do is a name for a Cajun dance party, originating before World War II. According to Mark Humphrey's notes from the Roots n' Blues CD "Cajun Dance Party - Fais Do-Do", the parties were named for "...the gentle command ('go to sleep') young mothers offered bawling infants." He quotes early Cajun musician Edwin Duhon of the Hackberry Ramblers, "She'd go to the cry room, give the baby a nipple and say, 'Fais do-do.' She'd want the baby to go to sleep fast, 'cause she's worried about her husband dancing with somebody else out there."
'Do-do' itself is a shortening of the French verb dormir (to sleep), used primarily in speaking to small children. Comparable to the American English "beddy-bye", it is still commonly used by French-speaking people.