Yes, feeling uncomfortable when someone, including a family member like your dad, slaps your bum is a normal and valid response. Physical boundaries are an essential aspect of personal space and respect for individuals. It's crucial to recognize that your body belongs to you, and no one should touch or engage with it in a way that makes you uncomfortable or violates your boundaries.
Physical contact like this can be seen as inappropriate or even abusive, depending on the context and intention behind the action. It's essential to communicate your feelings clearly and assertively to your dad about how you feel when he does this. If you don't feel comfortable having this conversation directly, you can consider talking to another trusted adult, like a family member, teacher, counselor, or friend, who may be able to support you or help address the situation.
Remember, everyone deserves to feel safe and respected in their personal space, and setting boundaries is an essential part of maintaining healthy relationships. If you ever feel unsafe or believe your boundaries are being violated, don't hesitate to seek help and support from someone you trust or a professional counselor.