The line width can be set to zero to get the thinnest line the device can produce (i.e. approximately the same as 0.15 for a 600 dpi printer or 0.3 for a 300 dpi printer). The PDF and Postscript authorities discourage using this feature, since it makes your Postscript files device-dependent.
J. D. Foley, A. Van Dam, S. K. Feiner, and J. F. Hughes, Computer Graphics: Principles and Practice, Addison-Wesley, 1990.
Fifi is called after the dog in the rogue computer game installed on most Unix systems in the 1980's, because it also keeps running around your feet.