To look out for number one is to put one’s own interests first, often at the expense of other people’s. It is usually said with a note of disapproval, describing someone selfish or coldly self-interested, though people also use it of themselves, half-joking, to justify a decision made purely for their own benefit. ‘Number one’ here stands for oneself, so the phrase is really ‘look out for myself’ dressed up as a piece of general advice or a confessed personal rule.
What’s the origin of the phrase ‘look out for number one’?
The slang use of ‘number one’ to mean oneself is older than the full phrase. Green’s Dictionary of Slang traces it back to 1704-5, in Thomas Pitt’s correspondence, later printed in Hedges’ Diary, where he writes of someone being “very careful of number one, and looks no further.” The word was evidently well established by the 1830s, since Charles Dickens could use it in Pickwick Papers without any need to explain it, having a character remark that “no man should have more than two attachments” apart from “the first, to number one, and the second to the ladies.”
The fixed phrase ‘look out for number one’, combining this slang sense of ‘number one’ with the older expression ‘look out for’, is attested by the 1830s at the latest, according to Wiktionary’s etymology. The earliest dated example found is from 1833, in an essay by George Washington Light published in The Essayist, a magazine aimed at young men. There the phrase is given as the stated motto of a cynical, self-serving character, who declares “Look out for number one, that’s my doctrine; and let others do the same,” before adding that anyone foolish enough not to follow the same rule deserves to be cheated.
That context fits the phrase as it is still used today. It was put into the mouth of a schemer who openly admits to serving only himself, and it has kept that slightly cynical flavour ever since, whether used as a criticism of someone else’s selfishness or as a wry admission of one’s own.