Local legend holds that the town of Festus, MO took it's name from Porcius Festus, a Roman government offical from the B.C. times.
Before it was incorporated as Festus, this town was unofficially known as Tanglefoot due to the large number of saloons in town. Festus is located adjacent to Crystal City, MO. Together, they are known as the Twin Cities of Jefferson County, MO. At the time, Crystal City was a "dry" town, so people went across the road to the "wet" township of Tanglefoot, which eventually became Festus.
Legend has it that a name would be randomly selected from the Bible, and Porcius Festus was the name selected. The rest of the legend is a bit dubious. The story goes that the townspeople agreed to fire a rifle at a Bible, and whichever name fell closest to the bullet's center, on the last page of the Bible which the bullet penetrated, would be the name of the town.
I personally doubt this story, as I'm sure that it was uncommon practice in the 1800's to shoot holes in Bibles. Anyway, the name stuck.
As to the other question, it gets very hot and humid around here in the mid summer. There are a number of colloquialisms to describe these weather conditions, but none of them are related to the town's name.
Steve.