I got this, again, from QT in the Chicago Sun Times, written by Jay Smith. At least I credit him/her; he/she has used our stuff before, which of course is complimentary.

Anyway, it seems a Brit wrote in and asked about the correct term for "hair's breadth." He has heard it as "hair's breath" or "hair's breadth" or hairsbreadth (in AHD) or hairbreadth. He also suggested hare's breath, as the hare evidently breathes shallowly. He said the OED didn't help him.

I don't use the term much, but if I were to, I would say "hair's breadth," I believe. According to Jay Smith, Shakespeare didn't even know. Jay cited these quotes:

"Of hair-breadth 'scapes i' the imminent deadly breach...."

"I see you are obsequious in your love, and I profess requital to a hair's breadth...."

Which is it?

I'd say either hair's breadth -- as in "He escaped by a hair's breadth" -- or hair-breadth as in "He had a hair-breadth escape".

I can't even understand why someone would say "hairs-breath," but indeed it is cited in Google.