Just like other poster mae says:

An attractive person of indeterminate gender who appears to have the "XXY" chromosome arrangement. Most often used to describe women who, though fashionable and made-up, look a bit manly.

To which I add:

Popularized by the They Might Be Giants song about hermaphrodites. Unfortunately , a lot of people who heard the song didn't actually listen to the lyrics, and think 'sexxy' is just a synonym for sexy.

An attractive person of indeterminate gender who appears to have the "XXY" chromosome arrangement. Most often used to describe women who, though fashionable and made-up, look a bit manly.

Used to emphasize the sexiness of someone. The more x's the sexier the person.

A purposeful misspelling of sexy originally created by the band They Might be Giants but brought into common use by geeks and nerds alike.

Previous definitions may be somewhat true, but if it were describing male tendencies, it would be SeXYY, not SeXXY. y is the male chromosome, not X.