This word is a whole cultural identity for some people, and it's impossible to see it written down without immediately thinking of Ireland, and perhaps of distant family ancestry too. Celtic tribes used to roam all over Western Europe, a warrior race with their own language and traditions who finally settled across Ireland and parts of Scotland.
Sports fans will be familiar with the word, too. Soccer fans will have heard of multi-time Scottish champions Glasgow Celtic, and all basketball fans know the Boston Celtics. So just how is it that, for all this time, we've been saying "sell-tick" when we should have been saying "kell-tic"? It's a hard one to follow - at some point, through being adapted into Latin, and then French, and then back again (having spent some time being pronounced, somehow, as "Tell-tic"), the hard 'c' at the start became soft.