What article should I use with the word sarcasm?

Question

What article should I use with the word sarcasm?

-Are you serious?

-No, it's [a/the/ ] sarcasm.

Answer 1

Sarcasm should be understood as an uncountable quality which is attributed to an utterance.

The word is not used in English (as it is in some other languages) to mean an instance of sarcasm. We do not, for instance, say

^∗ "How beautiful!" was a sarcasm.

If you want to attribute the quality of sarcasm to a particular utterance you may say:

No, "How beautiful!" was sarcasm.
No, "How beautiful!" was a sarcastic answer.
No, "How beautiful!" was meant sarcastically.

By the same token, if you speak of someone's sarcasm—

Fred's sarcasm offended me.

—you are not saying that the actual words he said offended you, but that the explicit or implicit tone of what he said offended you.

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^∗ marks a usage as unacceptable

Answer 2

It's sarcasm.

also note that this word in uncountable.

http://learnersdictionary.com/definition/sarcasm