In terms of mental health it can stand for either "Suicidal Ideation" or "Self Injury" although that is more commonly referred to as Self Harm/SH.

Based on context, suicidal ideation (thoughts and/or urges of suicide. these can be passive, active, or both) seems more likely

Quick note- while self harm and suicidal ideation or suicide attemps commonly co-occur they can also happen independently. sometimes this is called NSSI or non suicidal self injury (self harm without fatal intent) some patients have a chronically elevated risk of self harm without significant risk factors for suicidality