It's when you can see the full outline of a dude's bell-end through the fabric of whatever he's wearing on his undercarriage. Because this spectacle ALWAYS looks like an actual fish eye. This is the truest meaning of the term because it is so common and so hilarious. Fellas, you thoughts you'd pulled the wool over with your tales of jizzing in our eyes but look down next time you're wearing your softest jim-jams, and there he is, finding nemo.
NB: Fish eye is most notable in the circumcised community, it also features heavy in those belonging to the 'well-endowed' faction.
2👍 41👎