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1.
This is the male equivalent of yariman (which refers to female sluts) and is used to say that a guy is sleeping around in Japanese. A male slut.

"yari" comes from the verb "yaru" which can be used to mean "play".

"chin" is short for "chin chin" which is playful way to say "dick" in Japanese.

Literally it means a guy has or is a "playing around dick"

Double Standard: Unlike yariman which can be very derogatory word used to describe girls, "yarichin" has less impact and is often used as a joke.
"Ore ha yarichin chau de!!" - I'm not fucking around!!
(said in stronger Kansai dialect when, for example, a boy is arguing with his gf that he not sleeping around)
by PP Tokyo July 15, 2009
 
2.
The companion slang term to yariman - could be roughly translated as "manslut"

Derived from "yaru," to do, and "chinko," slang for dick. Used, I suppose, for guys who give it up.
That yarichin can't keep it in his pants.
by Bam City June 27, 2006