What does コスプレ Cosplay mean?

Cosplay is a Japanese word made out of English. It was coined by a specific person, in a specific year, after a specific event, which is unusual: most vocabulary has no birth certificate and this one nearly does.

The coinage

In 1984 the writer Nobuyuki Takahashi attended the World Science Fiction Convention in Los Angeles and was struck by the masquerade, the costume event American fandom had been running for decades. Writing about it afterwards for a Japanese magazine, he needed a term.

Masquerade was available and he rejected it, judging that the existing Japanese rendering carried the wrong associations: masked balls, disguise, concealment. What he had watched was people performing as characters, which is close to the opposite of concealment.

So he assembled one from costume and play, and the compressed Japanese form entered fan vocabulary from there.

Why play was the right half

Because it points at performance rather than clothing. A costume is a garment; playing is something a person does. The word claims that the practice is closer to acting than to dressing up, and that claim has held: the culture that grew around it cares about pose, expression and character, and treats the outfit as equipment rather than as the point.

English masquerade could not have carried that, which was Takahashi's argument.

The return journey

English then took it back. The word now appears in English dictionaries and is used fluently by people who have never encountered Japanese, describing an activity that predates the term by decades in the English-speaking world.

So English fandom was doing this before the word existed, and adopted a Japanese coinage to name its own long-standing practice, because the Japanese coinage was better. That is a more interesting history than either language usually gets credit for.

What the case demonstrates

That wasei-eigo is not a defective form of English. A Japanese writer looked at an English word, judged it inadequate for the thing in front of him, built a better one out of English parts, and the English-speaking world eventually agreed and took it.