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The horseman, taking in the meadow at a glance and seeing that there was nobody else there, took his ease and asked in a derisive tone, “Why did you bristle up? Where did you come from?”

He quickly looked around the meadow
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Hi,

I mean "why did you become alerted?" , "why are you acting agressively?"

The horseman is a stranger, which suddenly appeared in the meadow where a nomad with his young wife stopped for an overnight stay. Of course, the nomad, Chiledu, is suspicious of him, he holds his bow in his hand, ready to draw it at any moment, he answers unwillingly, abruptly.

'He bristled' is used more to describe a person's mental attitude than their physical one. It is also commonly in response to some remark by another peron. The word does not seem to me like a good choice here.

Perhaps-

'Why are you so suspicious of me?'

'Don't you trust me?'

Clive
Comments  
Hi,

The yellow part is fine.

But what do you mean by 'Why did you bristle up'?

Clive
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Hi, Clive,

Nice to see you here again) Thanks a lot for your help!

I mean "why did you become alerted?" , "why are you acting agressively?"

The horseman is a stranger, which suddenly appeared in the meadow where a nomad with his young wife stopped for an overnight stay. Of course, the nomad, Chiledu, is suspicious of him, he holds his bow in his hand, ready to draw it at any moment, he answers unwillingly, abruptly.

Liza
 Clive's reply was promoted to an answer.