My Concise OED at home has two further meanings for "purl":

1.
v. - turn upside down or upset;
n. - a heavy fall (as in, "to take a purler")

2. (I'd never heard of this one, believe it or not!)
n. - Ale or beer with wormwood infused, or hot beer with gin mixed in (ugh! to both mixtures - they can only be for medicinal use, surely?)