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.

When your so pissed off from losing multiple amount of games that you play worse and worse till your so pissed you begin leaning over your keyboard screaming over every little mistake you make and complaining about gaming imbalances

(Common in Starcraft II)

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.