on-deck

The term "on-deck" is a straightforward nautical compound. "On," a fundamental preposition of position, combines with "deck," a word borrowed into English from Dutch maritime vocabulary referring to a ship's floor or platform. Literally describing a location on a ship's main level, its logic was extended in American baseball during the 19th century. The baseball diamond was metaphorically viewed as a ship, and the next batter, waiting in a designated area, was said to be "on deck," leading to the modern abstract sense of being next in line or ready for action.

This is a compound adjective formed from the preposition "on" and the noun "deck". "On" is from Old English on, of Germanic origin. "Deck" is from Middle Dutch dec, meaning 'covering, roof'.