"Fàilte! Cò thusa?"
Translation:Welcome! Who are you?
I read where thusa is an emphatic form of the pronoun thu. In other words, it's like saying in English "Who are YOU?" ; what English does by vocally placing stress on a word, Gàidhlig does by modifying the pronoun. Also, I see that certain copula actually require the usage of this emphatic form, which is why you say "Is mise..." when identifying yourself and not "Is mi..".