A woman holding a memorial evening for her deceased partner was slashed at the event.
Laura Fowler organised the get-together at the Old Mail House pub, in Staple Hill.
But when Jansen Whittaker asked her why she was looking so upset an argument ensued, Bristol Crown Court heard on October 23.
Whittaker then went away but returned, grabbed Ms Fowler by her hands and cut one of her fingers with a small knife.
Whittaker, 40, of Sandringham Avenue in Bristol, pleaded guilty to assault occasioning actual bodily harm (ABH) and possessing a blade.
The recorder Mr Barnes jailed him for eight months.
He told Whittaker: "It was a quite deliberate and very unpleasant assault without any justification at all.
"Six stitches were needed in the finger and she was distressed."
Ms Fowler made an impact statement in which she said she had been left feeling anxious after the assault.
Whittaker made no comment when interviewed.
John Stokes, defending, said his client had not slept for five days before the offence.
He said Whittaker suffered from a depressive disorder and had been using crack cocaine before the incident.
Mr Stokes said: "He took umbrage in the argument with the complainant.
"He accepts he inflicted injury upon her."
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