The fuller answer
An intrusive thought is an unwanted image, idea, phrase, or urge that appears without being chosen. People can have thoughts that conflict sharply with their values, attractions, identity, or intentions.
Trying to prove that a thought means nothing can become a checking ritual: reviewing feelings, testing arousal, avoiding people, confessing repeatedly, or searching for certainty. The distress often shows that the thought is unwanted, not that it is a secret plan.
A thought is not permission to act, but thoughts that come with intent, planning, loss of control, psychosis, or immediate danger require urgent professional help. A clinician can distinguish anxiety-related intrusions from other conditions.