Something or someone that smashes, or something very impressive and successful (like 'that movie was a smasher').
From 'smash' plus the agent suffix '-er,' meaning 'one who smashes.' In British slang, also came to mean something that is very good or impressive, as in 'smash hit.'
Calling something a 'smasher' in British English means it's brilliant, but in American English we'd say 'hit'—it's wild how the same root word means different things in different countries.
Complete word intelligence in one call. Free tier — 50 lookups/day.