1
general::
1. Available, ready for action, as in We had ten kids on deck to clean up after the dance. [Slang; second half of 1800s] 2. In baseball, scheduled to bat next, waiting near home plate to bat, as in Joe was on deck next. [1860s] Both usages allude to crew members being on the deck of a ship, in readiness to perform their duties.
American Heritage Idioms
2
general::
Phrase(s): on deck
1. Lit. on the deck of a boat or a ship. • Everyone except the cook was on deck when the storm hit. • Just pull up the anchor and leave it on deck. 2. Fig. ready (to do something); ready to be next (at something). • Ann, get on deck. You’re next. • Who’s on deck now?
McGrawhill's American Idioms And Phrasal Verbs