From an Old English female personal name Mildþrӯð, derived from mild ‘gentle’ + þrӯð ‘strength’. This was the name of a 7th-century abbess, who had a less famous but equally saintly elder sister called Mildburh and a younger sister called Mildgӯð; all were daughters of a certain Queen Ermenburh. Their names illustrate clearly the Old English pattern of combining and recombining the same small group of name elements within a single family. This name was in fairly regular localized use until the early 18th century and it enjoyed a strong revival throughout England in the 19th century. Its use declined from the 1930s onwards.
French: from Old French teil ‘lime tree’, ‘linden’ (Latin tilia), hence a topographic name for someone who lived near a lime tree, or a habitational name from a place named Thil or Le Thil (of the same origin), in northern France.