I'm not a fan of making up excuses like "I have a dentist appointment". However, I am a fan of being firm yet vague enough make it seem like that: "I have an arrangement after work" is perfectly servicable and most times wouldn't net follow up questions and is also not lying the least. The arrangement might be you spending time in front of the telly at home - that is a valid thing to have planned. You just don't have to explain it.

If asked further, you can still keep it vague with "It's a personal matter" because...well, it is. It's your personal time alone. Anybody pushing further is a little bit creepy and can rightfully be told that. If you want to be less abrasive yet firm a "Sorry, but I don't appreciate you peeking into my personal life/affairs like that" works if it comes to that.