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 judicial review into the project's funding has been dismissed, allowing the Championship side to press ahead.
"It's like an early Christmas present," Groves told BBC Radio Cornwall.
"The good news means that we can now crack on at the beginning of next year with getting the refinancing [of the club] organised."
Pirates have been attempting to make progress with their proposed 6,000-capacity ground for several years, but it has been complicated by Cornish football side Truro City also wanting to build a new stadium.
Asked when work on the prospective stadium will start, Groves added: "That's the question - my guess is sometime in the middle of next year.
"The first stage is refinancing and recapitalising the Pirates, the second is around the stadium design and building a sustainable business, and the third phase is around a rugby partnership."
The rugby partnership is with Super Rugby franchise Waikato Chiefs, whose chairman Dallas Fisher is visiting England in January.
Groves says there could be potential for player swaps with the New Zealand side in future.
"That is an idea going forwards, probably a couple of years down the track," he said. "I would describe it more like IP [intellectual property] swap - coaches, and maybe one or two players.
"The first thing we've been focusing on at the moment is steadying the ship. I don't think people realised how bad the financial situation was at the end of September - the Pirates were pretty close to going into bankruptcy or administration."
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.
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Cornish Pirates director Colin Groves has estimated that work on the Stadium for Cornwall will start "in the middle of 2017".