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He hadn’t expected the backhand, only because he’d lost himself in his own rage.
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Zane is being phyically abused by Graham. A backhand here means a blow that is struck with the back of the hand.

I usually see it in the form 'a backhander'.

Clive

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Backhand here refers to insulting, negative remarks, delivered forcefully.

It is a short form of the phrase "backhanded remarks."
See entry #3.

https://www.thefreedictionary.com/backhanded