Dillan Connal, 23, "justified" the attack to police by telling officers the man owed him money for Valium, a court was told.
Stirling Sheriff Court heard that Connal, 23, had met Christopher Dench, whom the court was told was "in his 60s", in the street outside Mr Dench's home in Kerse Gardens, Falkirk, in the summer of 2020 and this led to them having "sporadic contact".
In early November 2020 Connal began to assault Mr Dench in his own living room "for no apparent reason".
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Kristina Kelly, prosecuting, said: "Christopher Dench would be slapped and punched on the head and on occasions would be strangled."
Ms Kelly said Mr Dench believed Connal wanted to intimidate him and "instil fear".
She said Mr Dench estimated he was assaulted "every other day" by Connal, who would let himself in and out of his house without permission, having been understood to have obtained a key for the property.
Ms Kelly said: "The accused seemingly assaulted Mr Dench with his trouser belt, which he removed from his trousers.
"He would strike Mr Dench on the legs, mainly with the leather section of the belt - which was of thick, dark brown leather with a heavy buckle.
"Mr Dench could not provide an apparent motive for the attacks, other than the accused trying to instil fear and assert control over him."
About 6.30 pm on November 25 as Mr Dench was returning from collecting a prescription from his local chemist, he was approached from behind by Connal who told him, "You better get the money for me".
Ms Kelly said Mr Dench was unaware what he owed money for.
He was then punched on the face several times by Connal and fell to the ground before realising that money he'd had in his pocket was no longer there.
He reported the matter at Falkirk Police Station, where officers observed he had injuries "across his entire body", including cuts and swelling to the face, bruising to his arms and back and marks and bruising on the legs. On one occasion he'd had to have stitches, and had also been treated for a minor head injury.
Connal was arrested and seemed "irate and extremely angry" and told cops he'd given Mr Dench drugs and he owed him £200 as a result.
Ms Kelly said: "He shouted that he'd slapped Mr Dench on the face but justified this by saying it was because he'd given him Valium on tick and he hadn't paid for it."
Appearing by video link, Connal, of Camelon, Falkirk, pleaded guilty to assaulting Mr Dench to his injury on repeated occasions between November 1, 2020 and January 2, 2021, and assaulting and robbing him.He also admitted assaulting and robbing a 30-year-old man, Alexander Young, of money in Mr Young's home in Falkirk on Christmas Day 2020, and assaulting a third man, described as the new partner of Connal's ex-partner, at a house in Bonnybridge, Stirlingshire, on June 11, 2021.
Sheriff Keith O'Mahony said: "Having regard to the nature of these offences and Mr Connal's record there's simply no other way to deal with this other than a custodial sentence."