The story
From cook + -ing. The noun and adjective follow from the verb. The use of the word cooking to describe cheap bitter was popularized by the title character of the BBC sitcom Oh No, It’s Selwyn Froggitt (1974-1978). It refers to cooking sherry, a kind of sherry used in cooking but not suitable for drinking on its own.
Etymology from Wiktionary, CC BY-SA 4.0.