on

 1. Creating disadvantage for. (In numerous expressions, e.g. The car broke down on me.)

 2. Willing and ready.

   ♣ Are you on?

 3. Possible.

   ♣ It's just not on.

 4. Payable by.

   ♣ The drinks are on me.

 5. Wagered on.

   ♣ We'll have £1 on the favourite.