John Brown landed the blow in the face of Charles Johnstone, who then struck the back of his head on the ground, after trouble flared at a taxi rank.
Brown, 25, fled the scene, leaving the attack victim lying unconscious, but was later found by police crouching behind a fence in the rear garden of tenements in Dalkeith, Midlothian.
Sentencing him yesterday at the High Court in Edinburgh, judge Lord Uist told Brown that the punch he delivered must have been “one of considerable force”.
Mr Johnstone, then 48, suffered a skull fracture and haemorrhage, and surgeons operated to remove blood clots and reduce swelling.
Advocate depute Andrew Brown QC said: “He was required to remain in a medically-induced coma because of the issue of increased pressure on his brain.”
Mr Johnstone required further surgery and spent almost a month in intensive care before he was transferred for rehabilitation.
The prosecutor said the attack victim was unable to return to his work as a factory manager and was made redundant. He added that he still suffered physical weakness, poor balance and headaches.
Brown, of Stoneybank Gardens North, Musselburgh, East Lothian, last month admitted assaulting Mr Johnstone to his severe injury, permanent disfigurement and to the danger of his life on December 18 last year.
The joiner and delivery driver had been out drinking with his uncle, while Mr Johnstone was also drinking in Dalkeith with work colleagues.
Brown left the pub after midnight and went to a taxi rank in the town’s Lothian Street. Mr Johnstone and his colleagues also arrived there.
Brown later claimed to police that inappropriate remarks were made and he lost his temper after a further derogatory comment was made.
The advocate depute said Brown tried to punch one of Mr Johnstone’s colleagues, but failed to land a blow.
He said Mr Johnstone moved to protect his colleague, but Brown turned towards him and struck him full in the face with his right fist.
The victim then hit the ground, where he lay unconscious.
Brown was taken to Dalkeith police station and said: “I hooked the boy.”
He admitted the assault and running away afterwards after being told to do so by his uncle when he was interviewed by detectives.
The prosecutor said that when Brown was informed of the seriousness of the injury he became “extremely upset”.
He said: “I’m really sorry. I apologise to him and his wife, his family and I wish him a speedy recovery.”
Mr Johnstone was taken to Edinburgh Royal Infirmary where his injury was assessed as life threatening before he was transferred to the city’s Western General Hospital.
The advocate depute said Mr Johnstone was left with extensive, permanent scarring as a result of the operations he went through.
Defence counsel Ronnie Renucci told Lord Uist that nine times out of ten the victim of such an assault would have got up off the ground suffering nothing more than sore jaw.