A guy who has little guy syndrome or small guy anger. Schmaus is a blend of the words Mouse and Schmuck. A Schmaus usually wears really tight shirts and will drive a big vehicle. The word describes someone that is small and due to his size he is angry at the world. He will either try to become rich or pretend to be rich. He feels taller standing on his (imaginary) wallet. A Schmaus gets really irritable around bigger guys.
That guy is driving that big truck because he is either a Schmaus, has a small penis, or both.