John Harrington, 51, said he slapped Terry Hutchinson, 64, and it was not malicious but "playful".
Mr Hutchinson fell to the ground at West Herts Golf Club and suffered a brain injury, it is alleged.
Mr Harrington, of High Street in Redbourn, denies manslaughter at Luton Crown Court.
Mr Hutchinson had moved to Thailand where he married and returned to visit friends.
He was treated at the scene and taken to St Mary's Hospital in London, where he died on 27 May last year after three days of treatment.
Prosecutor Robert O'Sullivan QC said: "Mr Harrington did not intend to kill Terry Hutchinson. He is charged with manslaughter.
"The prosecution say he deliberately and unlawfully assaulted Mr Hutchinson and that assault was a significant cause of his death."
Mr Hutchinson had been drinking in the Red Lion in Bushey before being driven to Mr Harrington's pub, The Bull in Redbourn, the court heard.
Later another friend joined them and they were taken to the golf club where members of Mr Harrington's extended family were attending the wedding reception.
Mr O'Sullivan said: "There was a fair bit of banter on the part of Mr Harrington.
"What may have started out as drunken high spirits concluded with the defendant assaulting Mr Hutchinson with such force he went to the ground immediately and did not get up."
Mr Harrington told police: "I think I slapped him. I don't think I punched him. It was an open hand, not a fist. It was a playful slap."
The case continues.
A pub landlord killed an old friend with a single punch on the way to a wedding reception near Watford in Hertfordshire, a court has heard.