Is "JJ" slang for "penis" in Chinese?
In Mandarin 雞巴/Jībā/ is slang for 'penis', And it is often further nicknamed 雞雞/jī jī/ or 小雞雞/Xiǎo Jī Jī/ 

JJ is a shorthand for 雞雞/Jī Jī/

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A possibility for Cantonese to also use jj for penis:

In Cantonese, we call children's (or small) penis 賓周
Since 周 sounded like 啫, we also call it 啫啫 or 啫啫仔
JJ sounded like 啫啫, therefore is used instead in some casual Cantonese publication, mostly entertainment news or gossip column.

As for adult (or big) penis, there are several different names for it.