What does "on ice" mean?

In reserve or readiness. This idiom often occurs with put, meaning “to place in reserve,” as in Let's put that proposal on ice until we have the funds to implement it. This usage alludes to putting things in cold storage for preservation. [Slang; late 1800s]

In prison, as in He's been on ice for ten years. This usage may be derived in part from the slang term cooler for “jail.” [Slang; c. 1930]

With a good chance of success, as in I'm sure she'll win—it's on ice. [Slang; early 1900s]