Pensioner Andrew Connan thought he was being a good citizen when he waved at a car to tell the driver to put its headlights on.
But instead of flashing the 78-year-old a 'Thank you', the car stopped and two teenagers got out – and beat the living daylights out of the old man.
Christopher Graney and his girlfriend Samantha Fenton pushed Andrew to the ground and repeatedly punched and kicked him.
The couple, both 19, were last month convicted of causing actual bodily harm by a jury at Croydon Crown Court over the incident in Bromley, South-East London last October.
Yesterday, they were each jailed for two and a half years.
Mr Connan was waiting for a bus home after a night out when he gestured towards a car being driven erratically by Graney, because it was dark and the vehicle's lights were not switched on.
Fenton asked her boyfriend, who had been drinking: "Are you going to let him get away with that?"
The pair then got out of the car and attacked the pensioner.
Judge Stephen Waller told the couple: "You knocked him to the ground - and he was obviously a defenceless, elderly man - and you repeatedly kicked and punched him.
"You kicked him and hit him so hard with your fist it caused an injury to your right hand, and you were seen to step back as if to take a flying kick and changed your mind.
"I have seen the photos of the victim's injuries and at the time they seemed very serious. An attack on an elderly man could have led to very serious injury."
Although Andrew has now recovered from his injuries, after reading his victim impact statement the judge said: "His life will never be the same again. There was a vulnerable victim and it was a sustained assault and neither of you showed the slightest degree of remorse for this shocking attack.
"Only a custodial sentence is justified, and one that reflects the court's and public's abhorrence at such an attack on an elderly man."
Graney will serve an additional six months for a similar attack in which he smashed pedestrian Jayden McKenzie over the head with a metal drain cover. That earlier assault took place in August 2011 in the forecourt of a petrol station in Beckenham, and led to the victim requiring hospital treatment for a head wound.