A euphemism for "penis" popularized by Hiroyuki's Fate/Stay Night doujin Platonic Magician H (プラトニックマジシャンH).
For the original scene in the doujin, see post #27032. Saber says "p〇n〇s" (オ〇ン〇ン, ochinchin) while talking to Shirou. Shortly after, she also says "vag〇〇〇" (オ〇〇〇, omanko) and "JAM IT IN".
Links
- http://knowyourmeme.com/memes/jam-it-in-you-lost-me