The Collaborative International Dictionary of English v.0.48:

Amma \Am"ma\, n. [LL. amma, prob. of interjectional or imitative
   origin: cf. Sp. ama, G. amme, nurse, Basque ama mother, Heb.
   ?m, Ar. immun, ummun.]
   An abbes or spiritual mother.
   [1913 Webster]