The name is a combination of the greek prefix proto ("first" or "before", "original") and the element actinium, because it is the precursor of actinium in the so called decay chain of uranium.
Whenever a radioactive element decays, it loses some of its components (protons and neutrons) and becomes a completely different element. This element in return can also be radioactive and unstable and may decay again turning into another element. This process continues until eventually a stable element (more precicely: isotope) occurs. This cascade of elements transforming into one another is called decay chain. In this case it is the uranium-actinium-series, which decays in twelve steps from uranium-235 via thorium-231, protactinium-231, actinium-227 all the way to lead-207.