Thump insensitive bigmouth (8)

I believe the answer is:

blowhard

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'bigmouth' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?

'thump insensitive' is the wordplay.
'thump' becomes 'blow' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'insensitive' becomes 'hard' (associated in meaning).
'blow'+'hard'='BLOWHARD'

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