see a man about a horse

Definitions

(Expression) a term to prevent an explanation or reason for departure (which usually comes after a child or another adult asks a parent or an other adult where he or she is going, and is a polite way of saying, 'it is none of your business').

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(Expression) To excuse oneself to go to the toilet. Possibly a more specific meaning for "It's none of your business," but generally understood by listeners.