What does ツンデレ Tsundere mean?

A tsundere is a character who is hostile on the surface and attached underneath, and whose hostility is usually a defence against admitting the attachment. The word is built from the two states and the switch between them.

The two halves

ツンツン (tsuntsun) describes being aloof, curt, standoffish; the sort of person who turns their face away. デレデレ (deredere) describes being soppy, doting, visibly smitten. Both are mimetic words, a class of Japanese vocabulary that describes manner and state directly rather than through metaphor, and Japanese has a great many of them.

The compound is recent. It emerged from Japanese message-board fandom in the early 2000s, around 2002, as shorthand for a pattern people had noticed in visual novels and dating games. It is a fan coinage that escaped into general use, not a traditional term.

What the label actually requires

Two things, and the second is what most usage forgets. There must be a hostile surface, and there must be a specific reason the character maintains it. A character who is simply mean is not a tsundere. A character who is mean because being kind would expose something they are not ready to expose is.

This is why the archetype ages badly when it is written lazily. Handled well, the hostility is characterisation: it tells you what the person is afraid of. Handled badly, it is a dial the writer turns for comedy, and the audience is asked to find repeated cruelty endearing because affection is coming later.

The criticism worth taking seriously

The pattern normalises the idea that hostility signals hidden interest, which is a poor thing to teach and a worse thing to act on. The strongest version of the defence is that the trope is about the character's fear rather than about the recipient, and that well-written examples make the cost visible: the tsundere is not charming for being cruel, they are trapped in a posture they cannot drop.

Both readings survive contact with the material, and which one applies depends entirely on the writing rather than on the archetype.

Why English keeps the loanword

Because English can describe the behaviour but has no name for the type. Prickly, standoffish, hard to get all point at pieces of it, and none of them carries the crucial part, which is that the coldness is load-bearing and temporary. The word arrived attached to a whole pattern, and English had the pattern but no label for it.