The Colloquy in the November 1976 issue of Word Ways mentions the curious fact that buttermilk is milk from which the butter content has been removed. This makes buttermilk the opposite of what it ought to be, judging by its name: milk containing butter.
Xixx, Jezebel Q.
Word Ways: Vol. 10
, Article 7.
Available at: https://digitalcommons.butler.edu/wordways/vol10/iss1/7