Source : Webster's Revised Unabridged Dictionary (1913)

Sis \Sis\, n.
   A colloquial abbreviation of {Sister}.

Sis \Sis\, n.
   Six. See {Sise}. [Obs.] --Chaucer.

Source : WordNet®

sis
     n : a female person who has the same parents as another person;
         "my sister married a musician" [syn: {sister}] [ant: {brother}]