According to Hanks & Hodges' A Dictionary of First Names
the name Casilda is of uncertain origin, first prominently born by a saint in the 11th century who was born in Toledo, Spain, and who is believed to have been or Moorish descent.
According to Gutierre Tibon's Diccionario etimologico comparado de nombres propios de persona
, Casilda was derived from Germanic Hatuhild
, going back to words that meant "combat" and "battle." However, Jose
Albaiges Olivart's Diccionario de Nombres de Personas
says that Casilda was originally Arabic and derived from a word meaning "to sing". I would assume that both of those are just educated guesses, which is why Hanks & Hodges list Casilda's origin as being uncertain.