The word '69' often refers to a sexual position involving two partners who align their bodies in opposite orientations, allowing for mutual oral-genital stimulation. The number is used as slang to symbolize this act because of the way the digits visually represent the positions of the partners.
The term '69' draws its name from the shape of the numerals, which depict the positioning of the two partners in the act, where one is upside down relative to the other.