Swindon Magistrates’ Court heard Robin Outlaw, 56, was out for New Year's Eve with his partner when the couple had argued.
Prosecutor Keith Ballinger said the man had gone to another pub for a few hours but returned to see his partner and their friends at the Carpenters’ Arms in Sherston.
They had argued again and he slapped her across the face. Paramedics were called and, in turn, called police. Outlaw admitted the offence when he was interviewed, said he was sorry and it shouldn’t have happened.
Solicitor Ian Kelcey said his client thought his drinks may have been spiked.
He said: “He had not had that much to drink and his reaction seemed to be a very adverse one.”
He was still in a relationship with the victim of the assault. They had been living together since magistrates lifted his bail conditions in March.
Outlaw, of Easton Square, Sherston, admitted assault by beating. Magistrates fined him £288 and ordered he pay £117 in costs and surcharge.