Japanese work for penis. Variations of the word include chinko, or chinchin.
All of these variations can be regarded as juvenile euphemisms i.e., terms generally used by, or for the benefit of children - similar to the words "pee pee" and "wiener" used in America, or "willie" used in the UK; however, in Japan, these terms are also commonly used between adults.
The formal Japanese word (the dictionary equivalent of penis) is "Inkei"; however, this word is almost never used in conversation - even between physician and patient, the word "ochinko", or one of its variations is used.
How do you say "John's penis is humongous" in Japanese? You say it like this: "John-san no ochinko wa hijooni dekkai desu".