How aggressive might baby be? (2,2,4)

I believe the answer is:

up in arms

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I'm afraid I can't even guess how this answer relates to the clue.

(Other definitions for up in arms that I've seen before include "Protesting vigorously, ready for fight" , "Weapons expert should be" , "Involved in protest" , "Better prepared for battle" , "Prepared to protest strongly" .)

I've seen this clue in the Sydney Morning Herald.

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