Yokachinchin (よかちんちん - which can translate into good d***) - is a silly folk jingle that is sung by kids and some adults as a playful dig towards others. It's almost like a beer song or playful taunt.
The lyrics can vary but typically they go something like this:
♪ひねればホッ、ひねればホッ、ひねればひねれば、っあーーあいい気持ちー
俺のちん○ん、よか○んちーん!!♪
Heneba ho, Heneba ho, Henereba Henereba, ah, ah ie kimochi, ore no chinchin, yokachin
Hold it, touch it, Hold it, touch it...oh it feels real good, look at my good/big d***, look at good d***
Here are some examples of variations of the song:
Not sung all that commonly although perhaps if drunk some old men may sing it in jest.