U

Under starters orders
When the official starter of the race is satisfied that all the runners in the race are at the start (in flat races, in the stalls) and ready to race, a flag is raised signalling that the field is under orders and 'off'. Bets on any runner failing to start after this signal, are lost.

Underdog
The team or player considered by the bookmaker to have less chance of winning than their opponents.

Up the arm
Slang for odds of 11/8.