My grandfather was involved in the naming of both of these rather large Japanese companies as a VP when they were trying to cross over to the U.S. and I have heard the stories hundreds of times so I figured this is a good venue to share them.
Panasonic (Originally Matsushita) actually got the name of their company from a review of one of their speaker systems. The article said that they had great "all around sound." All around translates to pana and sound translates to sound.
Epson (Originally Seiko) made a small printer named the EP-101 which was the worlds first compact, lightweight digital printer. My grandfather found large demand in it in the U.S. so they needed to create a new name to use(Seiko is a watch corp in the US). He told them the story about how Panasonic came about the name and left on a flight back stateside.
When he got back, he had a message waiting already and they told him that they were naming it Epson. He told them that is a horrible idea because people would confuse it with epsom salt. They told him it was his fault because it was his idea and explained that they were naming it based on their first product sale like panasonic did. So the name comes from "son of EP" to the more consumer friendly Epson.