A man seen carrying two knives in a Carluke street claimed he was only going to cut his brother's birthday cake.
Brian Yule later hurled abuse at police officers and threatened to slit their throats.
Yule, 45, a prisoner, appeared via videolink at Airdrie Sheriff Court this week.
He admitted illegal possession of knives and threatening or abusive behaviour on March 14.
The court heard Yule was drunk when he arrived at the Co-op in Main Street, Carluke, around 3pm.
He accidentally knocked over a bottle in an aisle and it smashed.
Staff noticed two kitchen knives sticking out of his trousers pocket and escorted him outside.
The police were alerted and officers found the accused in Hamilton Street around 3.40pm.
Lauren Cole, prosecuting, told the court: "The knives were found in his pocket and he was arrested.
"In the police cell van he banged his head repeatedly off the walls, causing a head injury.
"He said the knives were to cut his brother's birthday cake.
"Yule was taken to hospital for treatment to his head injury and en route he was shouting and swearing at the police officers.
"He threatened to kill them, saying, 'when I get out of jail you'll get it.'
"He pointed at the officers and ran his finger across his throat in a slitting gesture."
Defence agent Kevin McCarron said Yule, who admitted previous convictions, had fallen out with his girlfriend.
He had been drinking and went to the Co-op for more alcohol.
Mr McCarron stated: "He takes medication and should not be drinking. His recollection of that day is extremely vague.
"However, the knives he had were not used or even referred to by him.
"He is anxious to put this type of behaviour behind him and find employment on his release from prison."
Sheriff Fergus Thomson jailed Yule for a year, reduced from 18 months because of his guilty pleas.
The sentence was backdated to March 15 as he had been held in custody since then.
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