What does a falling out among thieves mean?

Answer 1

Dictionary.com says a "falling out" is "a quarrel or estrangement between persons formerly in close association with one another."

"A falling out amongst thieves" is one (whether metaphorical or literal) that could be hazardous to those falling out, as both sides would have potentially harmful information about the other.

For instance, in the example set forth by the phrase, the thieves could turn each other in to the law. Since obviously neither side sees eye-to-eye any longer, both sides must consider that possibility. To take the analogy even further, one side might attempt to forcibly and preemptively prevent the other from doing so, either by turning them in first, or through more violent means.