1.
dress; one-piece dress
An informal abbreviation of ワンピース (one-piece), referring to a dress — a single garment covering the torso and extending down over the legs.
A clipped form of ワンピース (one-piece), itself a Japanese-English word that refers specifically to dresses. In English, "one-piece" typically means a swimsuit, but in Japanese fashion, ワンピース and its abbreviation ワンピ always refer to dresses.
This abbreviation is casual and used mainly among young women in everyday conversation and fashion contexts. The full form ワンピース is used in more neutral or written contexts.