What does ワイフ Waifu mean?

Waifu is the English word wife, pronounced through Japanese phonology and written in katakana. English speakers then borrowed it back with a meaning it never had in Japanese. It is a loanword twice over, and the round trip is what makes it interesting.

Why it sounds like that

Japanese syllables are built from consonant-vowel pairs and the language has no standalone f as English uses it. A borrowed word gets reshaped to fit: wife becomes ワイフ, three morae, wa-i-fu. The same process gives コーヒー for coffee and テーブル for table. Nothing about the transformation is specific to fandom; it is what happens to any English word entering the language.

The scene, and what it did not do

English speakers met the word through Azumanga Daioh in 2002, in a moment where a teacher drops a photograph, is asked who the woman is, and answers in English. The delivery is what carried it: an unremarkable statement made faintly unsettling by who is making it.

The common claim is that this coined the term. It did not. The loanword had been in ordinary Japanese use for decades, meaning a wife, somewhat dated in register. What the scene did was introduce it to an English-speaking audience already inclined to collect Japanese vocabulary, in a context involving a photograph of a woman and a man's disproportionate attachment to it.

The meaning English invented

From roughly the late 2000s the word settled into its current fandom sense: a fictional character, usually female, that someone declares a particular devotion to. That meaning exists nowhere in Japanese. A Japanese speaker hearing waifu hears wife.

The equivalent term in Japanese fandom is different vocabulary entirely, and the English word's usefulness comes precisely from being slightly foreign. It marks the claim as playful. Calling a character your wife in plain English sounds unwell; calling her your waifu signals that everyone involved knows it is a joke, mostly.

Why the round trip matters

Loanwords normally travel one way and drift in meaning as they go. This one left English, sat in Japanese for decades unchanged, returned through a single scene in a comedy series, and acquired a meaning on arrival that neither language previously had.

The mechanism is worth noticing because it is not translation. Nobody rendered a Japanese concept into English here. English speakers heard their own word said back to them in an unfamiliar accent and decided it now meant something else.