Within a gnat’s ...

Q From Doug Dew: A friend of mine uses a colourful phrase: within a gnat’s ..., meaning very close-fitting. Any ideas on the origin of this term?

A Various phrases of the type have been known in the US for at least 160 years to indicate something very small. The first example I found is cited by John Lighter in the Random House Historical Dictionary of American Slang from 1840: gnat’s heel, a very small amount. Others are gnat’s eyebrow, gnat’s ass (“Fine enough to split the hairs on a gnat’s ass”), and fit to a gnat’s heel, for something that fits or suits perfectly. There’s also the English gnat’s piss for any weak and unsatisfying drink. Others exist, some even more crude.