Putting metal in a microwave, e.g (4)

I believe the answer is:

nono

(Other definitions for nono that I've seen before include "Something totally unacceptable (informal)" , "Non-starter (colloq.)" , "Something that would be totally unacceptable" , "Impossibility" , "Something not acceptaable" .)

I've seen this clue in the Universal.

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