Think (one's) shit doesn't stink vulgar slang
If someone thinks their shit don't stink, they think that they are better, more powerful, greater, etc. than others.
She has acted like her shit doesn't stink after she became the monitor.
The verb "think" should be conjugated according to its tense.
Seem to be very suspicious or morally unacceptable
This deal stinks to high heaven. Maybe somebody was bribed.