IdiomCenter.com

get / have ants in one’s pants

1.
to become/be nervous and agitated — проявлять нервное возбуждение; быть взбудораженным; проявлять нервозность; (сидеть) как на иголках

Lucy always gets ants in her pants before a test.

I wonder if all actors get ants in their pants before they go on stage.

You have ants in your pants today. Is something wrong?

see also
[butterflies in one's stomach]
[be on tenterhooks]
2.
to be so excited or full of energy that you are unable to sit or stand still (used humorously) — [шило в одном месте]

Jack can not sit still; he has ants in her pants.

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Refers to the agitation caused by ants crawling into one’s pants.