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.
people often do irrational or foolish things when they are in love.
I and my boyfriend have decided to get married in the summer. I don't think that's wise, since we won't have enough money saved by then to set up house. But then, one cannot love and be wise.