"You are stupid, Iain."
Translation:Tha thu gòrach Iain.
Iain is technically in the vocative case here - the name just doesn't show it. Names beginning with a vowel don't get the a vocative particle, and masculine names slenderise the final consonant where possible, usually by adding an i just before the last consonant/consonant group. As Iain (and e.g. Alasdair and Eairdsidh) starts with a vowel and already has a slender final consonant, no changes are necessary to change the name to the vocative case. There is also no change for masculine names that begin and end in a vowel, e.g. Anndra.