There are plenty of guesses on the internet about where the phrase ecky thump comes from.
As I am a Brit not from southern england, I will break it down for you.
Eck E Thump
Heck ( hell or exclaimation ) ‘e ( He ) Thump ( punch )
Fuck that hurt
Ouch he punches
A similar phrase might be thats gotta hurt or that smarts.
Basically a humorous exclamation of shock or awe.
