There's a Keijo character who's ethnically brown with a rock-hard ass when she flexes, and she's nicknamed "Vrajass"/"Vajrass". I don't know what the reference/joke is besides presumably some Indian god maybe?
金剛 (kongou) is the Japanese term for diamond (though diamond is more common in modern days) or material incredibly hard, taken from the Sanskrit word "vajra" which can mean either thunderbolt or diamond.
It is also a weapon in Hindu and Buddhist legends with both lightning and diamond (or basically the hardest material) properties, though Japanese usually makes this distinction, calling the weapon 金剛杵 (kongousho), diamonds 金剛石 (kongouseki). The Keijo!!!!!!!! name 金剛尻 replaces the last character with the character for "butt", and whatever translation you saw probably took the opportunity to combine "vajra" with "ass". Note when she attacks, an image of one of the 金剛力士 (kongo rikishi)、阿形 (agyo) is shown in the background, as in in Buddhism, he wields a 金剛杵, or vajra. (Pic of Agyo here for comparison)
It also ties in with the the joke when the announcer says 南無阿弥陀仏 (namu amida butsu) when Yoshida falls, as that is a Buddhist phrase to send off someone after death.