a stubborn, dense, or otherwise thickheaded Irishman or person of Irish descent.
I was a stubborn toddler; Grandpa would try to get me to wash my hands before dinner, but I would refuse...my hands had not dirt on them, why wash?
"Jesus Christ, you are one thick mick, ya know that?" queried a frustrated Grandpa.
(aye, this is how grandpa spoke to toddlers)