assimilated form of L. ad- used before r-, as in adrogantia, arrogantia, arrogance. Reduced in OF. to a-, which in 14th c. was often re-spelt ar- after L., and so usually in Eng. from 15th c.; hence most words from OF. in ar- are now written arr-, e.g., arrange, array, arrive. In 16th c. this spelling was erroneously extended to words with a- from other sources: as a(r)raise, a(r)reach, a(r)rear. See AD- 2.
