Means “plank”, from Old French, itself from Late Latin planca. This could have referred to a person who lived by a plank bridge over a stream, someone who was thin, or a carpenter.
From the Roman cognomen Florianus, a derivative of Florus. This was the name of a short-lived Roman emperor of the 3rd century. It was also borne by Saint Florian, a martyr of the 3rd century, the patron saint of Poland and Upper Austria.