I didn't until I ran across it recently. Well, I guess that's sorta the way all words are learned. "I didn't know about it until I knew about it!" Ha ha.
Anyway, it means "physically beautiful" (usually associated with female outward beauty) although I would think it could apply to waterfalls as well.
After I looked it up, I suspected there might be a derivative noun. At first I thought "pulchritudite" but it is really "pulchritude". There is also "pulchritudity" interestingly enough.
A quick (Safeseach off) google image search suggests that "pulchritudes" has become a sophisticated euphamism for soft-core artistic porn.
Dial H for Hips That Won't Quit
3 minutes ago