Growing up I always used to term "fish on" when I actually had a fish on the line while fishing. As of late, I hear more and more people use it similar to the way you would use "carry on." To be honest, this bugs me and seems to be a misuse of the term. I see this a lot in the comments sections of websites/videos. Here's an example.
Blah, blah, blah. Your video is the best! You da man.
Fish on!
I've always used the term to alert my companions that there is a fish on the end of my line, and they all use it in the same way. If somebody on my boat says "fish on" I will usually pull my line in and ask if they need assistance (net, etc.)
How do you all use this term, or do you use it at all?
