In oaths, birr means fulfillment and ḥinth means a breach. When an oath concerns an act that is normally impossible in its apparent meaning, its operative sense and legal effect require separate analysis.
A well-established figurative usage may provide an interpretable meaning, but a remote resemblance alone cannot supply the legal subject of a binding oath.
Uttering an oath, giving it a legally cognizable meaning, and finding that expiation is due are three distinct stages. They should not be conflated.