sis

Short definition from Wiktionary

Noun

1. 1
   Clipping of sister.
2. 2
   Initialism of second impact syndrome.
3. 3
   Initialism of snow immersion suffocation.
4. 4
   Initialism of student information system.
5. 5
   Initialism of supervised injection site.
6. 6
   Acronym of share-in-savings.
7. 7
   plural of SI

Proper noun

1. 1
   Initialism of Secret Intelligence Service.
2. 2
   Initialism of Schengen Information System.