Etymology
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sis (n.)

1650s, abbreviated form of sister; in American English, applied generally to girls and young women (1859). It also was the familiar short form of Cecilie, Cicely, a common name for girls in the Middle English period.

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Definitions of sis

sis (n.)
a female person who has the same parents as another person;
Synonyms: sister
From wordnet.princeton.edu