noun

• (Britain) Initialism of Secret Intelligence Service. [(UK, formal) MI6]
• Initialism of Schengen Information System.
• Initialism of second impact syndrome. [A rapid and severe brain swelling caused by a second concussion before a first has sufficiently healed, such as may occur after suffering two concussions during the same sports match. It has catastrophic results and may cause death or permanent disability.]
• Initialism of snow immersion suffocation.
• (education) Initialism of student information system.
• 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.]
• Acronym of share-in-savings. [(US) A government form of performance-based contracting that moves risk to industry.]
• (informal) Clipping of sister. [A daughter of the same parents as another person; a female sibling.]