To cause to cook or to cause to undergo heating slowly at or below the boiling point.
To be on the point of breaking out into anger; to be agitated.
To remain angry with someone or something past the point of exhaustion; to resign oneself to holding a grudge, especially after some failed attempts to resolve a situation.
Someone who plays a sim (a simulation game), particularly The Sims.
Origin
From alteration of dialectal simper, from Middle English simperen ("to simmer") , of possibly imitative origin. First attested in the intransitive sense. The noun is from the verb.