adverb
Dated adverb; in modern Japanese, 早く or 速く are far more common. Still encountered in older literature or fixed expressions.
彼はとう立ち去った。
He quickly left.
noun, noun which may take the genitive case particle 'no'
Dated noun/adjectival use; often appears in the phrase とうの昔 (とうのむかし) meaning 'long ago' or 'ages ago'. Modern equivalents include ずっと前 or とっくに.
それはとうの昔に終わった話だ。
That's a story that ended a long time ago.
Standard modern adverb for 'quickly'; とう is dated and literary.
Standard modern adverb for 'swiftly'; とう is dated and literary.
Common modern adverb for 'long ago'; とうの昔 is a dated equivalent.
Derived from classical Japanese; the exact historical derivation is uncertain. The kanji 疾 carries the meaning of 'quick' or 'rapid', but the word is now almost exclusively written in kana.