This spawned a whole subthread elsewhere along the lines of "I'll just convert to an image format where nobody can dispute what the acronym means", which spawned another subthread of "JPEG isn't even an acronym unless you say juh-feg", but staying on the first subthread I looked up what WebP stands for and that's not documented anywhere.
I've been assuming WebM is "web movie" and WebP is "web picture".
Someone said they thought P in WebP is for "predictive". I don't think so.
VP8 uses (as do other video codecs) P-frames, predictive or predicted frames which are deltas to the frames before them, and I-frames, intra frames, which are complete images. VP8 does not have bidirectional B-frames. I guess this is where "P is for 'predictive' " came from, but WebP does not use P-frames, WebP is based on VP8 I-frames.
Maybe the P comes from the P in VP8? I have not been able to find an explanation for what VP3/4/5/6/7/8/9 stands for.
The announcement, the documentation, the Wikipedia article, neither of them explains the WebP name.