Definition of "on deck"

on deck

prepositional phrase
1. (nautical) On the deck of a ship, especially of a captain or officer; in charge.
  Quotations
  In every sense of the expression he is ‘on deck’; but my Jim, for the most part, skulked down below as though he had been a stowaway.
  1899 September – 1900 July, Joseph Conrad, chapter 19, in Lord Jim: A Tale, Edinburgh, London: William Blackwood and Sons, published 1900, page 213
2. (baseball) Being the batter who will be up next, typically when that player is waiting on the field.