"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.
(said in stronger Kansai dialect when, for example, a boy is arguing with his gf that he not sleeping around)