The story
From the appearance of the hat as playfully reflecting female reproductive tract anatomy and also evoking cat's ears, in a double entendre of pussy (“female genitalia”) and pussy (“cat”).
Etymology from Wiktionary, CC BY-SA 4.0.
pussy hat
Today it means: A simple pink knitted hat playfully reflecting a double entendre, worn in symbolic solidarity with women. Introduced for the Women's March on January 21, 2017, and subsequently worn at various protest marches and rallies. Full definition →