Green’s Dictionary of Slang

1. a deckhand.

1984

Partridge DSUE (8th edn) 297/2: from ca. 1800.

2. a deck passenger.

1931

N. Van Patten ‘Vocab. of the Amer. Negro’ in AS VII:1 28: decker. W. n. A deck-passenger.

3. (Aus.) the top deck of a double-decker bus.

1999

(con. 1979–80) A. Wheatle Brixton Rock (2004) 66: ‘How you getting home?’ ‘Er, by decker, innit.’.