(n.) someone who changes their opinion to the direct opposite of their previous viewpoint
That’s Halkerston’s cow, a’ the ither way is an old Scots proverb first recorded in paremiological collections dating from the nineteenth century. It describes a person or situation in which someone reconsiders their opinions, before turning their viewpoint around to the opposite of whatever it was before.
At the root of this saying is an apparent legal wrangle between a “lawyer and landed proprietor” named Halkerston, and a man who had lawfully grazed an ox on Halkerston’s land.
The man initially told Halkerston that while grazing his ox, it had killed one of Halkerston’s heifers. Immediately concluding where the guilt lay in the case, Halkerston explained to the man that “your ox must go for [i.e. act as replacement for] my heifer.” But when it was explained to him that it was in fact the opposite that had occurred, and that Halkerston’s heifer had in fact gored the grazing ox to death, he immediately backtracked. “The case alters there,” Halkerston concluded, and instantly reversed his decision.
Quite how true that story is, of course, is questionable, but it nevertheless has curious parallels to an earlier expression—“The case is altered,” quoth Plowden—that emerged in the emerged in the eighteenth-century. The subject here is a Tudor scholar and lawyer named Sir Edmund Plowden, who according to one version of the tale was similarly approached by one of his neighbours seeking legal advice, after some pigs had escaped and trampled their way across his property. As an experienced lawmaker, Plowden began to explain to the neighbour that he could indeed seek compensation from the owner of the pigs—but on learning that the escaped pigs were in fact his own, Plowden concluded that “the case is altered”, and immediately tried to shirk all legal responsibility. As a result, the proverb “The case is altered,” quoth Plowden fell into use whenever some new or apparently game-changing information came to light.