A 71-year-old man was sitting in a chair catching his breath on the way to the shops when Ned Francis Amies knocked him unconscious.
The elderly man, who had brain surgery a month earlier, saw Amies trip on a footpath.
He asked if he was OK, but was answered with two punches to the face.
Amies was under the influence of methamphetamine, causing him to be "dangerously violent" and delusional, Judge Michael Byrne told the Brisbane District Court.
Thinking the elderly man was some sort of danger to an imaginary female associate, Amies lashed out.
The man lost consciousness and suffered a facial fracture and cuts that needed 13 stitches.
Judge Byrne said Amies was "probably very lucky" not to be facing a charge of homicide or grievous bodily harm.
Amies pleaded guilty on Tuesday to the serious assault of the elderly man at Margate, north of Brisbane, on September 20.
Just 23 days earlier Amies was released on parole for two other violent attacks.
He punched a 56-year-old woman known to him, later telling police she was interfering in his life.
The following day Amies attacked a high school acquaintance during an argument.
Judge Byrne sentenced Amies to 15 months in prison and ordered he be immediately eligible for parole.
But the judge said this was "nothing even remotely close" to an assurance Amies would shortly be released from jail.
"It is a decision for the parole board," he said.
Australian Associated Press
