call a cab

(raise arm while riding): From the similarity to the raised-arm gesture used to summon a taxi.

call a cab (third-person singular simple present calls a cab, present participle calling a cab, simple past and past participle called a cab)

1. Used other than figuratively or idiomatically: see call,‎ cab. (summon a taxi)
2. (horse racing) To raise one's arm to maintain balance while riding.
   • 1994, Dick Francis, Wild Horses, page 137:

     An habitually raised arm, termed 'calling a cab', was the trademark of amateurism.

   • 2002, Frank R. Scatoni, Finished Lines, page 181:

     Drawings of races sixty years ago show the riders trying to avoid catching their horses in the mouth by stretching forward with one arm and lifting the other hand backwards off the reins into the position known as 'calling a cab'.
     Although it is no longer the normal and recognized thing to do, 'calling a cab' is still occasionally seen when a jockey finds himself in difficulties over a fence.