Your insulated tumbler started out as lab equipment. In 1892 the Scottish scientist James Dewar built a flask from two glass vessels, one inside the other, with a vacuum between them. He needed it to keep liquefied gases extremely cold. The vacuum slows heat moving in or out, which is why the same design keeps coffee hot.
Dewar never patented it. In 1904, two German glassblowers, Reinhold Burger and Albert Aschenbrenner, saw it could keep drinks warm as well as cold. They gave it the name Thermos and patented it. Dewar's original flask is still on display at the Royal Institution.
The name caught on so widely that in 1963 a U.S. federal appeals court ruled "thermos" had become a generic word. That's why people say "thermos" no matter who made the bottle.