If a part of someone’s body is malformed, it is badly formed — it did not grow in the usual shape. This is the adjective that sits next to malformation: the noun names the problem; malformed describes the body part. It is a careful, medical word. You might hear “a malformed kidney” or “a malformed finger.” People also use it, a little more loosely, for objects that came out the wrong shape, like a malformed tool; the body meaning is the one learners meet first. It is not a rude word for a person. Say the part is malformed, not that the person “is a malformation.”