John Spears (60) attacked Gary Oliver in the bar in the Railway Tavern in Falkirk in March last year.
Mr Oliver had earlier been annoyed that Spears was chatting to his mum in the pub.
Police later traced the thug who blurted out: “It was me.”
Spears now faces jail after he today (Monday) pled guilty to assaulting Mr Oliver to his severe injury, permanent impairment and to the danger of his life.
A judge continued his bail pending sentencing next month.
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The High Court in Glasgow heard Mr Oliver had been out celebrating his mum’s birthday.
They later went to the Grahams Road pub.
Spears was also in the bar and he was spotted with Mr Oliver’s mum.
Prosecutor David Taylor said: “This interaction appeared to upset Mr Oliver, who was heard shouting: ‘He’s dead. I’m going to kill him’.”
At last orders, Spears, also of Falkirk, was still sitting with the woman.
He then got up with a glass in his hand and walked towards Mr Oliver.
Spears put his arm around him and smashed the tumbler on the bar.
Mr Taylor said: “Spears held the now broken glass up at Mr Oliver’s face and neck.
“He then drove it into him striking him on the right hand side of the neck as Mr Oliver struggled to free himself.”
The incident was caught on CCTV.
Bar staff intervened before Spears was put out the back door of the pub.
A 999 call was meantime made for the bleeding victim.
Mr Oliver was clutching a rag on a wound to his neck as police turned up.
He later told officers someone had “plunged” a glass into him.
Spears was traced later that morning elsewhere in Falkirk.
He immediately told police: “It was me. They called me a paedo.”
Mr Oliver needed treatment for a neck wound and has been left scarred for life.
Lady Stacey deferred sentencing for reports.