To deform is to force something to have a new shape by pushing or twisting it. While some kids love to shape animals out of clay, others prefer to deform their friends' elephants and puppies.
In physics or engineering, to deform something is to change its shape by applying force. You're more likely to come across the word deformed, used either as an adjective or a verb: "My cat's paws are deformed—see, she's got two extra toes on every foot." The source of this word is the verb deform, or "disfigure," which comes from the Latin deformare, "put out of shape."