c. 1300, "to make dark;" late 14c., "to become dark." The more usual verb in M.E. was simply dark, as it is in Chaucer and Shakespeare, and darken did not predominate until 17c. The Anglo-Saxons also had a useful verb, sweorcan, meaning "to grow dark." To darken someone's door (usually with a negative) is attested from 1729.