"Ambidextrous, adj.: Able to pick with equal skill a right-hand pocket or a left"

The quote by Ambrose Bierce is describing someone who is ambidextrous, or someone who has the ability to use both hands with equal ability. This could describe a variety of activities, but in this case, it is referring to the capability to select a pocket with either hand. This indicates that the person is competent in the art of thievery, as they have the ability to choose a pocket with either hand without any difficulty. This might be seen as a compliment to the person's ability, as it implies that they have the ability to choose a pocket with either hand with equivalent ability and mastery. It could also be seen as a cautioning to those who might be the target of such thievery, as it indicates that the individual is experienced enough to pick a pocket with either hand.