Having a sign language rap battle.

When you eat dirt for the first time and leave your family to build a shack in the woods

to be
derived from the latin "si" meaning "if"
alternative meaning: stupid idiot
as in brother-in-law to be ... if the stupid idiot ever proposes

Abbr. for 'stupid idiot'

S.I.S is an acronym for Special Investigation Section.

It is an Los Angeles Police Department's elite tactical detective squad with a straightforward mandate: Track down the City’s most dangerous offenders and take them off the street. S.I.S. are possibly the hardest-working, most dedicated men and women Detectives to wear a badge.

Means to "sit in silence" insetad of lol. Usually used if somebody sais something retarded online, its a good way to be honest. Lets face it very few actually laugh at stuff online.

Abbr. for "self-injury," also known as self-mutilation. Most common form is cutting, but may also include burning, bruising, skin-picking (excoriation), and other behaviors. There is no single cause for SI. SI is most common in women and tends to persist for years.