What does ヤンデレ Yandere mean?

A yandere is a character whose love for someone becomes dangerous to other people, and often to its object. The word joins 病んでる (yanderu), to be ill, with デレデレ (deredere), lovestruck.

Taking the word apart

病む (yamu) is the verb for falling ill. In the form 病んでる it describes a continuing state, and in ordinary modern usage it very often points at mental rather than physical illness. デレデレ is mimetic, one of the doubled sound-words Japanese uses in quantity, and it describes going soft and gooey over someone.

Ill, plus besotted. The compression is exact and slightly nastier than the English handling of it usually admits.

The half that gets misread

Because yandere and tsundere rhyme structurally, the yan can be mistaken for another temperament word, a stronger flavor of tsun. It is not. ツンツン (tsuntsun) describes being prickly or standoffish, which is a mood. 病んでる describes being unwell, which is a condition.

The distinction matters for reading the trope. A tsundere is hiding an ordinary feeling behind an ordinary defense. A yandere is not hiding anything; the feeling is stated and sincere. What has failed is the machinery that would normally keep a sincere feeling from becoming a knife.

Where the trope actually sits

It is worth being unromantic about this. The archetype is a horror device wearing a romance costume, and its usual function is to make possessiveness legible by giving it a body count. Stories that use it well know that; stories that use it badly present the violence as a measure of how much a character cares.

The word does not carry approval in Japanese either. Describing a real person this way would not be a compliment, and the illness in the etymology is not decorative.

The dere family, and how much of it is real

Fandom has produced a long list of terms built on the same pattern, describing temperaments with a dere suffix. Kuudere, from kūru (cool), is another that circulates in Japanese.

Why English needed the loan

English has plenty of words for dangerous attachment: obsessive, possessive, stalker, fatal attraction. What it lacks is a single term that holds the affection and the pathology together without deciding which one is the truth. Obsessive puts the weight on the pathology. Devoted puts it on the affection.

Yandere refuses to choose, and keeps both halves in the word where a reader can see the join. That is the shape of the hole in English, and it is why the loanword stuck when the available translations did not.