In the United Kingdom (UK), the term "chips" is used for potatoes that have been cut into long, narrow strips and deep fried. These fried potatoes are quite common and are sold very inexpensively in many takeout restaurants. The ready availability of this food at low prices has given rise to the idiom “cheap as chips,” a phrase indicating that the item being referred to is a very good deal and inexpensively priced.
Idioms are commonly used in the English language. They consist of groups of words that are used in a figurative, rather than a literal sense. People often use idioms without even thinking about them. Such phrases as “I heard it through the grapevine,” “Every dog has its day,” “Like two peas in a pod” and “Bite the bullet” add color to the language, but may be confusing to a non-native speaker, or to anyone who has not heard the particular idiom before.
In order to be able to understand an idiom, it is usually necessary for someone to understand the user’s frame of reference. A person doesn’t necessarily need to understand the full background of an idiom in order to be able to use it correctly, but if the idiom is to make sense, a bit of background is helpful. Knowing that chips are common and inexpensive in the UK helps to understand the phrase “cheap as chips.”
Some idioms are specific to a particular style of English. In the case of the phrase “cheap as chips,” the term chips means something different in UK English than it does in American English. In the United States potatoes cut into long, squared strips are called “french fries,” instead of chips. French fries are not particularly expensive, but they are not always synonymous with low cost. In this case, this particular idiom might be tough for someone who has never been exposed to UK English to decipher.
When the term “cheap as chips” is used, it doesn’t literally mean that the item in question will cost the same as an order of take-away chips, only that the item is reasonably priced and can be considered a good deal. “Cheap as chips” can refer to a car, a dress, a pair of shoes, a week-long vacation or just about anything else that you've obtained inexpensively.