What would offend a Celt?

[Cyclops]

What would offend a Celt?

Hide his menhir?

Kidnap his druid?

Tell him he is a mythical creation of Hellenic/Roman xenophobia compounded with 19th century sectarian/nationalist sentiment?

As there are no Celts today....

Cheeky fellow, there's lots of people who call themselves Celts.

They live in their ancient homelands of Brooklyn and possibly Queens, and worship at the shrine of the Gallo-Lenape deity Tammany

[roman]

Tell him that he is a part of "Fantasy" 2 hander sword unit, and is desperately trying to substitute lack of manhood with the size of the sword.

Plus i would guess a show of disrespect towards ones chosen deity might get things started nicely as well.

[ibleedgrenchese]

Just cut off his head stock it in his ass and throw him back to the gaul imsure someone might find that provoking

[russia almighty]

If he has a hot red head sister, banging her?