Is a man sinful if he touches another man's buttocks playfully?

It is not permissible for a man to touch from another man's body what he is not allowed to look at, even in jest. The prohibition becomes more severe if it is from the grave nakedness (awrah mughalladhah) or done with lust. Therefore, touching the buttocks is forbidden, and the doer and the one being acted upon, if he consents, both incur sin. Jest does not make permissible that which contravenes the Sharia.