thief the Good Thief a traditional name for the penitent thief, St Dismas.
set a thief to catch a thief proverbial saying, mid 17th century; in an epigram the Greek poet and scholar Callimachus (c.305–c.240 bc) has, ‘Being a thief myself I recognized the tracks of a thief.’ The saying is used to imply that the person best placed to catch someone out in dishonest practices is one whose own nature tends that way. (Compare an old poacher makes the best gamekeeper.)
when thieves fall out, honest men come by their own proverbial saying, mid 16th century; meaning that it is through thieves quarrelling over their stolen goods that they are likely to be caught, and the goods recovered. The shortened form ‘thieves fall out’ is used to imply that an association based on shared dishonesty is likely to be disrupted.

See also hang a thief when he's young, there is honour among thieves, little thieves are hanged, opportunity makes a thief.

thief / [unvoicedth]ēf/ • n. (pl. thieves / [unvoicedth]ēvz/ ) a person who steals another person's property, esp. by stealth and without using force or violence.

thief OE. þīof, þēof = OS. thiof (Du. dief), OHG. diob (G. dieb). ON. þjófr, Goth. þiufs; Gmc. word of which no further cogns. are known.