This is another “maraphor”. The speaker uttered this mix up, and then said, “I mean, keep your britches on.” This is a congruent conflation of “hold your horses” and “keep your britches (or pants) on”, both meaning to restrain yourself. Britches, or breeches, are pants used in riding horses so I can see where the speaker was confused. She probably visualized someone with breeches riding a horse. A big shout out to Marianne Julian who heard this and passed it on!