Derived in mod. Dicts. from Gr. αὐτός self + ὀνομασία naming, and explained as the use of a common name in a connection in which it acquires an accepted specific sense; e.g., ‘town’ for ‘London,’ ‘river’ for ‘Thames,’ in ‘He is in town, and has gone across the river (to Lambeth)’; it being held that ‘town,’ ‘river’ here virtually name themselves ‘London,’ ‘Thames.’ But as this is a strained etymology, and the word exactly corresponds in meaning to ANTONOMASIA, it seems more reasonable to suppose that it is a mere mistake due to a turned n (= u) in printing.
