mirth UK /mɜːθ/US /mɜɹθ/ noun2 •The emotion usually following humor and accompanied by laughter.But sorrow that is couch'd in seeming gladness Is like that mirth fate turns to sudden sadness. — 1602 •That which causes merriment.Phantasmal mirth, folded away: muskperfumed. — 1922 formsmirths