To be frank is to be honest. Also, it's a hot dog. Eating a frank at the ballpark is, to be frank, an all-American experience.
If you're open, honest, and candid, you're frank — that can mean refreshing honesty or too much information. Frank also has some lesser known uses. It can also mean to stamp with a postmark, or to pass someone through for free — like how your museum job lets you frank your friends into the planetarium. The most delicious meaning is for what you may know as a dog, frankfurter, hot dog, hotdog, weenie, wiener, or wiener-wurst.