S4i is the acronym of the term "safe for invite". S4i is used to indicate your displeasure of not being invited to a specific event or meeting. The term should be used with a hint of sarcasm, with the user knowing that there was a good and genuine reason for the lack of invite.

The abbreviated term "s4i" was initially coined by the London based Grudge Match XI Football Club in UK (2007); initially being used within emails between GMXI members, it's popularity then had the expanded term "safe for invite" being used verbally between GMXI members and is now used extensively within international communities.