Smashing is a BrE slang which means "very good" or "impressive". Most folks might know this already, due to its use as a catch phrase by various BrE characters in media.
However, from the usual meaning of the word smash (which means breaking with violence), it's not obvious how it could come to mean "very good". Did this meaning come from a specific context, perhaps military, or sports?
Best Answer
According to Etymonlime the term smashing has undergone the change in meaning like other terms such as: fabulous ( see below).
smashing (adj.):
Fabulous:
Probably its origin meaning 'impressive' comes from Tennis:
Smash:
A smash in tennis:
Source:Wikipedia