Try turning the handle in both directions as this can often loosen the seized part. Spray the penetrating oil down the shaft and where the gland nut is. If the stop tap handle will still not move then remove the gland nut using a suitable spanner, this will turn anti-clockwise.
why is it called stopcock?
Accordingly, why is it called stopcock?A stopcock is a form of valve used to control the flow of a liquid or gas. The term is not precise and is applied to many different types of valve. The only consistent attribute is that the valve is designed to completely stop the flow when closed fully.