If people can see through your shirt, it's sheer. This can also mean something steep, like a cliff, or anything extreme, like sheer nerve.
This word has several meanings, but they're all extreme. A sheer (see-through) shirt is an extreme way to dress — climbing a sheer cliff is almost impossible. If your car sheers, it swerves sharply, and you might crash. Maybe you've met all these meanings in a single nightmare: When the sheer cloth blew over your face as you drove, you lost control and the car began to sheer, filling you with sheer terror as you went over the sheer cliff.
Primary Meanings of sheer
so thin as to transmit light
very steep; having a prominent and almost vertical front
turn sharply; change direction abruptly