I don't think it means anything. It is a line out of a video by Rebecca Black titled Friday that has been spoofed by someone on the net. The front passenger in the convertible they are riding in the video is playing with the slipstream with her hand undulating as a porpoise would in the water, and yells "My hand is a dolphin!" (at least, in the overdubbed spoofed version)
<< Moderator note:
pops has very kindly provided a context in which this line appears.
It is starnightnt's responsibility to provide the context and background, so I hope we will soon have this confirmed (or corrected).
Wow! That's right. That's a phrase in which I heard on video clip titled Friday. I really don't know who spoofed her voice, but this phrase sounds like a nonsense stuff. Now I get it, maybe her hand pretended to be a dolphin. Thank you pops91710 and sorry panjadrum about not providing more context