I came across this word reading Robert Caro’s The Passage of Power. Volume four in his ridiculously thorough but amazing biography of Lyndon B. Johnson. He refers to a speech Johnson gave as a stemwinder.
The phrase originates from the name for a keyless watch, an invention from around 1875. Because these watches were so superior to their key-dependent brethren, any thing or person that was first-rate or energetic could be called a stemwinder. Sometime in the 1940s in political lingo, the phrase became particularly associated with rousing speeches and speech makers. A person or a speech could be a real stemwinder.