Repository shortened to its spoken name — one project's whole history and truth in one place.
Clipped from repository, from Latin repositorium (a place for storing), from reponere (to put back, to store): re- (back) + ponere (to place); a repo is a single place a whole project's history is put back into and kept.
Repository shortened to its spoken name — one project's whole history and truth in one place. The root's storing is unbroken by the clipping: everything is still put back into the same place, every time.