The camel's nose is a metaphor for a situation where the permitting of a small, seemingly innocuous act will open the door for larger, clearly undesirable actions.
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U.S. Senator Barry Goldwater used the metaphor in expressing his opposition to the National Defense Education Act in 1958:
This bill and the foregoing remarks of the majority remind me of an old Arabian proverb: "If the camel once gets his nose in the tent, his body will soon follow." If adopted, the legislation will mark the inception of aid, supervision, and ultimately control of education in this country by the federal authorities.
History
According to Geoffrey Nunberg, the image entered the English language in the middle of the 19th century. An early example is a fable printed in 1858 in which an Arab miller allows a camel to stick its nose into his bedroom, then other parts of its body, until the camel is entirely inside and refuses to leave. Lydia Sigourney wrote another version, a widely reprinted poem for children, in which the camel enters a shop because the workman does not forbid it at any stage.
The 1858 example above says, "The Arabs repeat a fable", and Sigourney says in a footnote, "To illustrate the danger of the first approach of evil habit, the Arabs have a proverb, 'Beware of the camel's nose.'" However, Nunberg could not find an Arab source for the saying and suspected it was a Victorian invention.
An early citation with a tent is "The camel in the Arabian tale begged and received permission to insert his nose into the desert tent." By 1878, the expression was familiar enough that part of the story could be left unstated. "It is the humble petition of the camel, who only asks that he may put his nose into the traveler's tent. It is so pitiful, so modest, that we must needs relent and grant it."
A 1909 essay by John B. West, founder of the West legal classification system, used the metaphor to describe the difficulty of trying to insert an otherwise innocuous set of facts into a rigid legal system:
three excellent digesters [] spent an entire day in disagreeing as to whether seal fishery cases should be classified under the topic 'Fish' or that of 'Game' .... It is the old story of the camel's head in the tent. What seems at first a plausible pretext for forcing some novel case or new principle into a topic or subdivision to which it does not naturally belong, leads to hopeless confusion.
In a 1915 book of fables by Horace Scudder, the story titled The Arab and His Camel ends with the moral: "It is a wise rule to resist the beginnings of evil."
The phrase was used in Reed v. King (193 CA Rptr. 130 - 1983): "The paramount argument against an affirmative conclusion is it permits the camel's nose of unrestrained irrationality admission to the tent. If such an 'irrational' consideration is permitted as a basis of rescission the stability of all conveyances will be seriously undermined." The case in question involved a plaintiff suing because the defendant sold a house without telling them that the house's previous inhabitants had been brutally murdered 10 years earlier.
Related expressions
There are a number of other metaphors and expressions which refer to small changes leading to chains of events with undesirable or unexpected consequences, differing in nuances.