Richard Wagner was a German composer in the 1800s. (In English, his last name is pronounced “vog-ner.”)
He’s most well-known for his epic opera cycle The Ring and specifically one of its musical numbers, The Ride of the Valkyries This was the music played through the helicopter speakers in the famous beach battle scene of Apocalypse Now.
Wagner was known for his grandeur and scope. It took him 26 years to complete The Ring, and the four parts would take about 17 hours if performed continually. Wagner intended for The Ring to be performed in a theater custom-made for it.
With this in mind, “Wagnerian” describes something of large and dramatic significance; something done on an epic scale.