I have no particular interest in manga and related phenomena—I don’t dislike them or disapprove of them, and I’ve seen some interesting examples, but life is too short to delve into everything—so I hadn’t been familiar with the term tsundere until I read a MetaFilter comment by Marisa Stole the Precious Thing (the username is a reference to a Japanese song whose original title is 魔理沙は大変なものを盗んでいきました Marisa wa taihen na mono o nusunde ikimashita); the comment explains:
In the older sense, “tsundere” was a character who appeared to be cold, aloof even arrogant, but over time developed a softer, more caring side. In the newer sense, it’s been used to mean a character who appears to be cold, aloof and arrogant on the outside but is actually, on the inside, filled with feelings of love and affection, usually for the character they’re coldest with.
You can learn more at the Wikipedia entry (which explains that the word “is a combination of the two words tsuntsun (ツンツン), and deredere (デレデレ)”); at any rate, the reason I bring it up is that MSTPT also linked to a video in which “Minoru from Lucky Star explains it quite well,” and it’s one of the best examples of word rage I’ve seen—at about the 1:20 mark, after discussing the change in meaning, he loses it: “I declare here, this is plainly a mistake! We must bring back the true meaning of tsundere and restore this depraved nation! Rise up, citizens!” Prescriptivism knows no national or generational bounds.