Your thumb is the short thick part on the side of your hand next to your four fingers.

The thumb of a glove is the part which a person's thumb fits into.

If you thumb a lift or thumb a ride, you stand by the side of the road holding out your thumb until a driver stops and gives you a lift.

If you say that someone or something sticks out like a sore thumb or stands out like a sore thumb, you are emphasizing that they are very noticeable, usually because they are unusual or inappropriate.

If you say that someone is twiddling their thumbs, you mean that they do not have anything to do and are waiting for something to happen.

If you are under someone's thumb, you are under their control, or very heavily influenced by them.