A contact in your phone that you have no memory about. Maybe some one you met on a night out and had a memorable feature about them that (at the time) you thought would trigger your memory the following morning.
John: "Hey who's this contact in your phone "Sexy Sally from the bar"
Chris: "Oh that's a Fish Pie Sue I have no idea who it is."
To have sex or casual relations with someone you work with. The consequences are risky; that you will be caught with 'company fish' or potentially loose your job, or just be stuck working in an awkward situation.... but you do it anyway.
"Damn, that boy is hot! Ima hit him up at the staff party!"
"Yeah he is, but I never fish off the company pier. After the last guy, we had a crazy fight in the lunch room later. I learned my lesson!"