By hook or by crook

'Hook' is a word with many meanings and as a consequence it appears in numerous English phrases - 'get one's hooks into', 'hook, line and sinker', 'on/off the hook', 'sling your hook' and, most notably, 'by hook or by crook'. That last phrase is one of the holy grails of etymology; many people are sure they know the derivation but, in truth, the origin is rather obscure. We can be sure that it is a very old phrase and that it was in general use by the late 14th century.
There may be examples of a form of the expression in the writings of John Wyclif from around 1380, but scholars aren't sure of their date. The first substantiated citation is from John Gower'sConfessio Amantis, 1390. :

What with hepe and what with croke they [false Witness and Perjury] make her maister ofte winne.

Gower didn't use the modern 'by hook or by crook' version of the phrase, but it is clear that he was using the reference to hooks and crooks in the same sense that we do now.

The earliest example of the modern usage of the phrase that I can find is in Philip Stubbes'The Anatomie of Abuses, 1583:

Either by hooke or crooke, by night or day.

As is my habit when the origin of a phrase is uncertain, I'll present the most commonly suggested theories and leave the rest to you:
Suggestion number one is that 'by hook or by crook' derives from the custom in mediaeval England of allowing peasants to take from royal forests whatever deadwood they could pull down with a shepherd's crook or cut with a reaper's billhook. This feudal custom was recorded in the 1820s by the English rural campaigner William Cobbett, although the custom itself long predates that reference.