Meaning of the Phrase ‘Honor Among Thieves’

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I'm not sure which one of these apparently flatly contradictory proverbs I heard first but I have definitely heard both several times. One of them is:

There is honour among thieves.

Another is:

No honour among thieves.

Do they simply mean the opposite of each other, therefore, at least in my mind cancelling each other out, or is there a subtlety here that I am not picking up on? Are they related, perhaps sharing an origin and then somehow becoming inverted?

Best Answer

The classic proverb holds that, "There is honor among thieves.”1

The meaning, of course, is the concept of "professional courtesy," that even the disreputable and unethical do - particularly among themselves - adhere to various sorts of moral codes of conduct.2

As to the converse, "no honor among thieves," the meaning is self-explanatory. Curiously, the concept isn't limited to the English-speaking world, as evidenced by this Spanish proverb: Piensa el ladrón que todos son de su condición. (The thief thinks that all are of his condition.)3

1 Early nineteenth century. "The Oxford Dictionary of Quotations," Fifth Edition, edited by Elizabeth Knowles (Oxford University Press, Oxford and New York, 2001). Page 612:11.
2 Also: "Honor is sometimes found among thieves." [Walter Scott]
3 http://www.elearnspanishlanguage.com/vocabulary/expressions/ex-proverbs.html