Joshua Paul Birtwistle, 21, of Punchbowl, pleaded guilty in the Launceston Supreme Court yesterday to wounding.

The male victim, then 18, fell over and his head hit the concrete footpath.

He suffered injuries including a five-centimetre long, jagged, deep laceration to his head and brain trauma.

Justice Robert Pearce noted Birtwistle's four convictions for assault and jailed Birtwistle for 15 months, backdated to August 13, when he was eligible for release from jail for other crimes .

Birtwistle will be eligible for parole after nine months and, upon release, will be subject to a previously imposed suspended sentence and probation order.

Justice Pearce said that Birtwistle's drunken, misguided and mistaken belief that the victim might have had something to do with what happened to his brother was no significant mitigation.

"This crime was a senseless act of violence committed by an intoxicated person in public against a stranger who had done nothing to provoke him, and who continues to suffer as a result," he said.

Earlier, Crown prosecutor Luke Brett told the court that about 11pm on January 26 this year, the teenage victim was sitting alone overlooking the Village Green, with the Main Street footpath behind him.

Mr Brett said Birtwistle asked a group of strangers if they knew who had stabbed his brother, and one jokingly said, "It was him" and pointed at the victim.

He said that Birtwistle raised a glass stubbie in his hand, said "You did it . . . you did it," and smashed the bottle on top of the victim's head.

The 18-year-old fell sideways and his head hit the concrete footpath.

Defence counsel Mark Doyle said that his client, who had had a traumatic childhood, apologised for his crime and wanted to return to his home state of Western Australia for work.