Thanks Jragon for digging up the original phrase.
The full phrase is: 任侠と書いて人魚と読むきん
Direct translation is: Write “任侠” (ninkyo = chivalry) and pronounce it as 人魚 (ningyo = merman/mermaid).
Except they aren’t really pronounced the same, they don’t even rhyme. I suspect it’s an intentionally bad pun (i.e. so bad it’s funny).