also: ぬすびとこんじょう
noun
thievish nature; cunning and greedy character
A yojijukugo describing a person's inherent dishonesty, slyness, and greed, often used in a critical or moralistic tone.
あの男の盗人根性には呆れてしまう。
I'm disgusted by that man's thievish nature.
盗人根性が染みついているから、何度注意しても直らない。
His cunning and greedy character is so ingrained that he never changes no matter how many times he's warned.
Similar meaning, but 泥棒 is a more common word for 'thief' than 盗人, making 泥棒根性 slightly more colloquial.
Compound of 盗人 (thief) and 根性 (nature, spirit). The reading ぬすっと is a euphonic change from ぬすびと.