More fully ex mero motu. Of one's own volition; without assistance from or recourse to a higher authority.
Origin
Mid 17th century; earliest use found in John Hacket (1592–1670), bishop of Coventry and Lichfield. From (with transposition of elements, probably after usual English word order) post-classical Latin motu mero (also with preposition ex motu mero), ablative of motus merus free impulse, volition from classical Latin mōtus motion, movement + merus.