To be timid is to be shy or careful because you feel afraid.
For example, you might feel timid on your first day at a new school, where your teacher and your classmates are strangers to you. Because you feel timid, you stay quiet, and you move slowly or carefully, afraid and unsure.
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If you try to pet a timid cat, it will run away, or even hide. It's scared of you, so it's careful not to get too close to you.