Green’s Dictionary of Slang

1. (prizefighting) to punch on the head.

1825

Pierce Egan’s Life in London 17 July 197/2: [A]fter a little toppering on both sides, the strength of the countryman enabled him to hold up his adversary .

2. to kill with a blow on the head.

1899

Boy’s Own Paper 10 June 582: They might topper us all so as to get our knives.