In Australian slang, an informer or person who betrays others to authorities; a grass or snitch.
From 'grass' (British/Australian slang for informer) + -er suffix (one who does); 'grass' itself rhyming slang from 'grasshopper' (copper/police informer).
Rhyming slang is like a secret code language—'grasshopper' became 'grass' became 'grasser,' showing how slang compresses and evolves faster than formal language.
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