人の褌で相撲を取る（ひとの ふんどしで すもうを とる） is to take risks with other people's money.

「人の褌で相撲を取る」（ひとの ふんどしで すもうを とる） is used disapprovingly to mean that you benefit yourself at someone else's expense or you make a profit by using other people's abilities.

「相撲」（すもう） is sumo or sumo wrestling, which is a Japanese style of wrestling, in which the people taking part are extremely large.

「褌」（ふんどし） is loincloth, a piece of cloth worn around the body at the hips by men, sometimes as the only piece of clothing worn. It used to be worn as Japanese men's undergarment made from a length of cotton. It is also used when you play sumo.