A stranger or a remarkable thing; very, extremely, remarkably; unusual, unfamiliar, strange, great.
Its inmates were few; in fact, it was rather apt to be empty: its occupants were usually prisoners for debt, or for some trifling breach of the peace, committed under the influence of the liquor that makes one "unco happy."
Charles Warner, Baddeck and That Sort of Thing. Amazon
He was as good at sending up pretentious black arrivistes as the white middle classes, and, like all surrealists, he knew how to shock by getting under the skin of snobs and hypocrites, sending up, like Robert Burns, "the unco guid".