あるまい=ある+まい means no, impossible.
1. [Conjecture of the negative result] [はずがない] can't...maybe; […でないかもしれない] maybe not...; [おおかた…でないだろう] probably not...
Example:
- たぶんそうではあるまい/Probably not like that.
- まだ雨は降るまい/It won't rain yet.
2. [Deduction of Negative Will] I don't intend to..., don't want to...
Example:
- 二度と行くまい/Never want to go again.
- うるさく言うまい/No more details.
So, you find that it actually has a sense of speculation in it.
It's not a direct negative tone.
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