To sheer is to swerve or turn sharply from a course. A car may sheer from its path to avoid hitting something in the road.
Sheer also means bright, thin, or transparent. A diaphanous gown is sheer.
In this second sense sheer can mean pure or unmixed. For example, someone may act out of a sheer desire to be helpful. In other words that person has absolutely no expectation of a reward for his or her act.