Badger to death
Harass or persecute.
The phrase 'badger to death' alludes to the nocturnal burrowing mammal Meles meles, that is, the badger. At first sight it would seem intuitive that the expression refers to the fate of badgers in badger-baiting, an erstwhile so-called sport in which badgers were pitted against dogs and the protagonists tore each other apart. However, those fights weren't as one-sided as we may now suppose. Badgers were chosen for this entertainment as they are extremely tenacious when cornered and have the ability to bite their prey until their teeth meet. This fact has led to the alternative view that 'badgering to death' originally referred to the fate of the dogs and meant 'killed by a badger'. We aren't ever likely to know which of these derivations is correct, although most etymologists favour the former explanation.
'Badgering' has been used as a verb to denote persecution for some time. Francis Grose gave a definition of it in the 1785 edition of his invaluable glossary A Classical Dictionary of the Vulgar Tongue:
Badger, to confound, perplex, or teaze.
The first record that I can find of 'badgered to death' in print doesn't refer directly to badgers, nor to fighting dogs, but is a metaphorical reference to theatrical performers. This reference is found in Charles Dibdin's journal of the dramatic arts The By-stander; or, Universal Weekly Expositor, 1790, in which he gives the following advice:
It is always worth a manager's while to engage a performer for three years. The first he is a drudge; the second he is a servant of all work; the third badgered to death, and at length dismissed.
Badger baiting was made illegal in the UK in 1835 and instances of it are now rare, but it still does go on and prosecutions are occasionally brought. Despite that decline, the phrase has been given a new lease of life in recent years. Many UK dairy farmers claim that badgers, which are carriers of Bovine TB, are responsible for spreading the disease and killing their cattle. Of course, tabloid newspapers usually report TB outbreaks with the headline 'cows are badgered to death'.