You probably fall well within the standard psychological definitions of a sane person, but it's possible nevertheless that, at some point, somebody might accuse you in German of being bonkers, nuts, cuckoo, psycho, mad, cracked, bonkers, potty, barmy, mental, unhinged, or just plain crazy. If you are familiar with a few of the German adjectives on the topic, you will be better prepared to react calmly and rationally, belying the accusation by the very coolness of your manner.
Sag mal, spinnst du?
Tell me, are you crazy?
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The verb spinnen in formal usage is the spinning of wool, but "are you spinning?" is a slang idiom for "are you crazy?"
Bei euch piept's wohl!
It's really chirping with you!
Caption 41, JoNaLu: Prinz DreckspatzPlay Caption
The verb piepen in its standard usage means to make a high, whistling sound like a bird, but bei jemandem piept es is a slang idiom for suggesting they are crazy.
Hast du eine Macke oder was?
Do you have a defect or something?
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The noun die Macke in formal usage is "defect," but in casual use eine Macke haben means to be crazy, to "have a screw loose" so to speak.
Some formal German adjectives referring to a loss of sanity include irrsinnig, psychotisch, geistig behindert, and geistig gestört. The term geisteskrank was a formal term in decades past, but is now considered outdated. As in English, there are very many informal or slang adjectives, including verrückt, wahnsinnig, irre, blödsinnig, blöd, and bescheuert, to name a few. Go to Yabla German and see how they are used in a real world context, but be careful how you use these words out there. The person you are accusing might really be crazy, after all!