Rosamund f. Old Saxon hros 'horse' + Old English mund 'protector', Old Saxon mund, Old High German munt 'hand, protection'.
The name of a 6th C Lombard queen and a 12th C mistress of Henry II.
Henry II's mistress was sometimes known as the 'rose of the world', reinterpreting the etymology of the name as Latin rosa 'rose' + Latin mundi 'of the world'.
The English vernacular form Rosamund is witnessed in 1321 in the matronymic byname Rosamund (EL-2 p. 121).