It's said to be the lowest form of wit (7)

I believe the answer is:

sarcasm

(Other definitions for sarcasm that I've seen before include "The use of irony to mock" , "Caustic remarks" , "Cutting wit" , "Ironically scornful language" , "The use of irony to convey contempt" .)

I've seen this clue in The Irish Times.

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