frot; frottage
Slang term for a sexual act between males involving rubbing penises together. The kanji 兜 (helmet) is a euphemistic reference to the glans.
They enjoyed frottage.
Compound of 兜 (kabuto, 'helmet', euphemistically referring to the glans) and 合わせ (awase, 'joining together'). The exact origin is uncertain, but the term is used in modern slang.