The phrase "unbuttoned shirt" refers to a shirt that is not buttoned up or fastened down the front. It means that the buttons on the front of the shirt are left open, creating an exposed or relaxed look. Full definition
The actor looks ready to party in his all - black Dolce & Gabbana ensemble, complete with unbuttoned shirt and jaunty fedora, at a photocall for Midnight in Paris. (people.com)
A woman begins unbuttoning her shirt in front of a man while trying to seduce him. (kids-in-mind.com)
Simply unbutton your shirt dress on the lower half of your body or add a belt. (fashion-landscape.com)