Thirty-nine-year-old Harold McDaniels Sr. says the attack was a "snap decision" he regrets.
Police say McDaniels was in his Ambridge apartment Saturday when he heard a neighbor yelling about an unrelated dispute, prompting an exchange of words.
Investigators say 22-year-old Keith Moffatt insulted McDaniels' weight and McDaniels slashed him. McDaniels was charged with aggravated assault and reckless endangerment while Moffatt needed surgery to close a wound from the left side of his jaw to his Adam's apple.
McDaniels said he feared for his safety when he grabbed the knife and slashed Moffatt.
Police say Moffatt was lucky to escape more serious injury.