This mysterious "lemon, outline" icon baffled me today. It stood out for being "lemon" as well as limited to just the outline version. I expected a matching non-outline version.

<http://fortawesome.github.io/Font-Awesome/icon/lemon-o/>

So, why does it exist this way (outline only)? Is the solid version some kind of taboo? Ultimately, in what kinds of contexts would one really make use a lemon icon? I'm puzzled, feeling as if I'm missing some sort of cultural reference.

To be clear, I'm not seeking a simple opinion. I'm looking for some concrete evidence of what that icon was originally intended for so that I can slot it into usage when the time calls for it. Since icons are a cultural artifact, I am seeking the wisdom of the Internet in an attempt to fill in the gap in my knowledge of what good a lemon might be for, as an icon.