The story
Likely inspired by a widely-parodied 2011 ad campaign "We're a Culture, Not a Costume," raising awareness of racially insensitive Halloween costumes.
Etymology from Wiktionary, CC BY-SA 4.0.
my culture is not your costume
Today it means: Within a fan group or subculture, an accusation of malicious appropriation by an outsider, or a denial of a person's credentials. Full definition →