noun
thievish nature; underhand character
Describes a person's dishonest, sneaky, or thieving disposition. Often used in criticism or self-deprecation.
彼は泥棒根性が染みついているから、信用できない。
He has a thievish nature ingrained in him, so I can't trust him.
そんなことを考えるなんて、自分
Even I hate my own underhand character when I think of such things.