Slang abbreviation of コスプレ. Used in casual conversation among fans to refer to dressing up as a character from anime, manga, games, etc.
See also: コスプレ
今度のイベントでコスするんだ。
I'm going to cosplay at the next event.
友達と一緒にコスの写真を撮った。
I took cosplay photos with my friends.
My friend was cosplaying as an anime character.
イベントにはコスで参加する人が多い。
A lot of people attend the event in cosplay.