Can be used as a question or statement of who is or should be next or aks the question of what's coming up.
This phrase is an old shipping term. It was used to find out who was or was supposed to be on guard, doing duties, etc. This term later moved into the realm of baseball and is still used to this day as a description of who's going to be batting next.
"Giambi is needs a double here in the sixth. If not, then the Yanks will have to count on Jeter whose on deck."
By Karrie
On Deck
to be the next or newest big thing. it comes from a double reference: it is a bastardization of "on point" which means on top of things, with a baseball on-deck circle. in baseball, the on deck circle is where the next batter stands while he waits for his turn to bat. hence, the next big thing is "on deck."