Tim's Grandfather is the unnamed grandfather of Tim. He isn't given a name, and appears in two BrainPOP episodes. He is Tim's paternal grandfather, according to the family tree in the Genetics episode.
He is the father of Tim's Father. He is also Tim's grandfather, and the son of Timothy, a World War I veteran. He and his family immigrated from Wales. He is a skilled chess player, and beats Tim at chess frequently.
He also has a very strong Welsh accent.
It can be seen he collects (or used to collect at one point) antique guns.