As a girl's name this is a pet form of Jacqueline, as in the case of Jackie Kennedy Onassis (1929–94). It was originally a boy's name, a pet form of Jack. The racing driver Jackie Stewart (b. 1939) was originally named John Young Stewart.
Spanish: habitational name from Las Mestas in Cáceres province, or Castilianized form of Asturian-Leonese Les Mestes, name of a town and several villages in Asturies, all named with the plural of mesta ‘common (sheep) pasture’ (from Latin mixtas, past participle (plural) of miscere ‘to mix’, ‘to mingle’).