To pull in one's horns

Another origin that I've been asked for, but can't give - this is getting to be habit!
ORIGIN of the phrase, 'to pull one's horns in'.
Any ideas?

I believe the reference is to snails, which when threatened 'draw in' their eyestalks (horns) in order to protect them.

DFG

Thanks for the help. I'd looked in Brewer, and that's what it says too, but I thought it a little too simplistic. However, it seems to be true. Thanks again.