This is a blunt way of shutting down an argument that rests on an impossible condition. Someone sets out a chain of reasoning that begins with “if only” or “suppose”, and the reply points out that adding an impossible physical detail to the situation would not change what actually happened, so the whole argument is pointless. YourDictionary defines the idiom as meaning that it is fruitless to speculate about counterfactual situations. It is deliberately crude, and the word “bollocks” is chosen because it jars, making the dismissal feel sharper than a polite version would.
What’s the origin of the phrase ‘if my aunty had bollocks, she’d be my uncle’?
The joke is much older than its vulgar British wording suggests, and it began life as a riddle rather than an insult. The earliest documented English-language version dates to the USA in 1808, phrased as “if my aunt had been my uncle, what would have been her gender?” The idea settled into a statement rather than a question, because by 1839 and 1840 several American newspapers, including the Norwich Aurora in Connecticut on 18 December 1839, were printing “If your aunt had been a man, she would have been your uncle.”
The joke was not confined to English. By 1843 a French equivalent already existed and was treated as an established saying, appearing in Eugène Sue’s novel Les Mystères de Paris with the line “if my aunt had been a man, she would have been my uncle, as the old proverb goes.” American sportswriting slang kept mutating the wording further: the Muskogee Daily Phoenix in Oklahoma printed “if your aunt had whiskers she’d be your uncle” on 6 August 1916.
The specifically vulgar British form, with the anatomical word rather than “been a man” or “whiskers”, was current by 1939. George Eric Rowe Gedye’s book Fallen Bastions quoted it that year, with the taboo word discreetly replaced by dots, as a diplomat’s rebuke to Lord Halifax: “If only your aunt had ● ● ● ● ●, she would be your uncle.” That printing shows the modern crude version was already established in British usage before it became a familiar piece of everyday slang.