Notes on The Holy Gospel of Jesus Christ, According to St. Matthew (Author Bishop Richard Challoner (ed.))
5:34 Not to swear at all: It is not forbid to swear in truth, justice and judgment; to the honour of God, or our own or neighbour's just defence: but only to swear rashly, or profanely, in common discourse, and without necessity.
Original Douay-Rheims from Project Gutenberg - Massive cleanup of OCR problems and editing done locally.