Rotundus is from Proto-Indo-European *reth₂- "run, roll".
In Celtic, *reth₂- is found in the combined form *to-wo-ret- "a running up to": *to- "to" and *wo- "under, up, up from under". This became Old Irish tóir "pursue", then *tóraighe "pursuer". This was borrowed into English as tory. The earliest meaning of tory is
1. a. In the 17th c., one of the dispossessed Irish, who became outlaws, subsisting by plundering and killing the English settlers and soldiers; a bog-trotter, a rapparee; later, often applied to any Irish Papist or Royalist in arms. Obs. exc. Hist.
2. With capital T: A nickname given 1679-80 by the Exclusioners (q.v.) to those who opposed the exclusion of James, Duke of York (a Roman Catholic) from the succession to the Crown.
3. a. Hence, from 1689, the name of one of the two great parliamentary and political parties in England, and (at length) in Great Britain.