noun
male masturbation
Vulgar slang term for male masturbation. Compare まんずり (female masturbation).
See also: まんずり
彼は毎晩せんずりをしているらしい。
Apparently he masturbates every night.
The origin is uncertain. The first element せん may be from 千 (thousand) used as an intensifier, and ずり is likely from 擦り (rubbing).