1a wine bottle of twice the standard size, normally 1 1/ 2 litres:a magnum of champagne
2( trademark in the US) a gun designed to fire cartridges that are more powerful than its calibre would suggest: that magnum has a kick like a field gun [as modifier]: a magnum bullet
Origin:
late 18th century: from Latin, neuter (used as a noun) of magnus 'great'