a) A sailing ship with two or more masts, all with fore-and-aft sails; if two masted, having a foremast and a main mast. There are variants, such as additional square sails on the fore topmast. Compare ketch and yawl which have a main and a mizzen mast.
b) A glass of beer. Size varies by state, but it is typically one of the larger measures, except in South Australia; see for details.
Wikipedia foundation.