adjective
crazy, extremely
B1
Wahnsinnig means 'crazy' (insane) or functions as an intensifier 'extremely' (e.g., wahnsinnig gut). It's a gradable adjective/adverb (Komparativ wahnsinniger, Superlativ am wahnsinnigsten) and opposite of vernünftig. Widely used colloquially for strong emphasis. Not reflexive and no auxiliary rules.
Examples
Er ist wahnsinnig kreativ und hat viele ungewöhnliche Ideen.
He's incredibly creative and has lots of unusual ideas.
Das Konzert gestern war wahnsinnig gut.
The concert last night was amazingly good.
Die Zuschauer waren wahnsinnig erleichtert, als das Spiel endlich endete.
The spectators were incredibly relieved when the game finally ended.
Details
Mnemonics
Picture someone so excited their hair is standing up — that extreme feeling is 'wahnsinnig'.
Sounds like 'wants-in-sick' — imagine someone wanting to be insanely energetic.
Notes
Used both as an adjective ('a crazy person') and colloquially as an intensifier meaning 'very' or 'extremely' before adjectives.