Definitions from Wiktionary ( )

▸ noun:  (British) Initialism of Secret Intelligence Service. [(UK, formal) MI6]
▸ noun:  Initialism of Schengen Information System.
▸ noun:  Initialism of second impact syndrome. [A rapid and severe brain swelling caused by a second concussion before the first one has sufficiently healed, resulting in possible death or permanent disability.]
▸ noun:  Initialism of snow immersion suffocation.
▸ noun:  (education) Initialism of student information system.
▸ noun:  Initialism of supervised injection site. [A medically supervised facility providing a hygienic environment in which people can consume illicit recreational drugs intravenously, to avoid drug problems such as disease and overdose.]
▸ noun:  Acronym of share-in-savings. [(US) A government form of performance-based contracting that moves risk to industry.]
▸ noun:  (informal) Clipping of sister. [A daughter of the same parents as another person; a female sibling.]

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