1. Dressing up is against my religious beliefs.
2. Dressing up is against my political beliefs.
3. My costume was destroyed in a house fire.
4. My costume was carried away by flying ants.
5. It's too cold to dress up.
6. It's too warm to dress up.
7. I came to this party straight from work, and didn't have time to get my outfit from home.
8. I came to this party straight from the airport, and didn't have time to get my outfit from home.
9. I came to this party straight from having a torrid affair, and didn't have time to get my outfit from home.
10. I lost my parents in a tragic Halloween-related accident, and I really don't want to talk about.
11. I was too ill with food poisoning to dress up.
12. I was too ill with bubonic plague to dress up.
13. I was too ill with regular plague to dress up.
14. I've come dressed as an average American / Brit / Canadian / Russian / Egyptian / Chinese / Nigerian / Scandanavian, and that's why I'm wearing normal clothes, you racist.
15. Why must we wait for a consumerist holiday to express ourselves? Why must society tell us when it is and isn't OK to dress up like a slutty nurse?
16. I'm dressed as a Muggle.
17. I'm dressed as a wizard who's actually quite good at dressing for the Muggle world.
18. Oh no, I thought Halloween was tomorrow!
19. But I thought we already did Halloween last year!
20. Sorry, I don't speak English.
21. Sorry, I didn't dress up for Halloween, because I'm not a child.
22. Sorry, I'm really pretentious.
23. I was dared not to dress up, and I never turn down a dare.
24. I'm wearing my costume under my regular clothes.
25. I was going to wear my costume, but it would have offended a lot of people.
26. I'm dressed as a teacher.
27. I'm dressed as a sinner.
28. I SHALL NOT ENGAGE IN PAGAN BLASPHEMY.
29. I'm not from around here – what's Halloween?
30. Halloween is an invention of the ruling classes to distract the masses with sweets and carved pumpkins so that we're not paying attention to like, the economy.
31. Um, you can't tell what my costume is? Hah, OK.
32. I'm not drunk enough to dress up for Halloween.
33. I'm too drunk to dress up for Halloween.
34. I can't afford to dress up for Halloween, so thanks for reminding me of that fact. Yeah, thanks a lot, thanks a million.
35. I don't celebrate Halloween by putting on a silly costume. I mark the triduum of All Hallows by solemnly remembering departed Christian saints and martyrs.
36. Ugh, Halloween is for dads.
37. I'm dressed as an alien who's trying to pass as a human being.
38. I haven't dressed up, but the billions of single-celled organisms that live in my body are all wearing tiny little witch costumes.
39. I'm dressed as an undercover police officer.
40. I'm dressed as a really great undercover CIA agent.
41. I'm dressed as love.
42. I'm dressed as you.
43. I was going to buy a costume, but then I used the money to donate to charity instead. How about you?
44. My costume is invisible. It's an invisible costume; that's the joke.
45. I'm actually wearing a costume on one of my internal organs – why should skin get all the fun?
46. I dropped my costume in the gap between the platform and the train.
47. I'm conducting a social experiment about not dressing up for Halloween. Don't tell anyone!
48. I dressed up as Angelina Jolie / Brad Pitt just hanging out at home on a Sunday.
49. I'm dressed as the miracle of life.
50. I'm dressed as the lead singer of a band you've probably never even heard of, but who are going to be huge this time next year.
51. I have extremely sensitive skin.
52. Better to not wear a costume than to wear what that person's wearing. *point at literally anyone*
53. Oh, it's an inside joke with that guy. *point at literally anyone*
54. Oh, I'm just about to go change into my costume! *walk away*
55. I fucking hate Halloween.