answer the call of nature

an evacuation of waste matter from the body.

Often described as:
Taking a piss or taking a shit

Bob: Excuse me I need to answer the call of nature.
*Bob gets up and walks away*
Jim: (to George) Where is Bob going?
George: To take a piss he said.
Jim: You sure he didn't mean take a shit
George: I could give a fuck less Jim.

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