The term “blow smoke” generally indicates a falsehood or trickery. It is commonly used in a phrase about blowing smoke up someone’s rear end to mean that a person is being overly flattering or telling a lie in order to get another person to purchase something or do something else in particular. The phrase is also related to other common phrases like “smoke screens,” which also means to conceal the truth.
Some claim that the term “blow smoke” was inspired by magicians who use smoke and fog to conceal their tricks. The term has been around for many years, and it was often used to refer to corrupt sales people who peddled merchandise which did not deliver on the promises made by salesmen. It is also used when referring to car salesmen and other modern-day “snake oil” sales professionals who make overly hyped claims in order to make money.
Others claim that the concept of blowing smoke up one's backside originated from a method of treatment for patients requiring resuscitation. It is said that a rectal tube was inserted and smoke from burning tobacco was allowed to enter the body. This was supposed to cause the patient to take a breath or regain consciousness. These claims were obviously false and unsubstantiated, thus giving rise to the term "blowing smoke."
Many times, to “blow smoke” refers to flattery. This could be from a salesperson offering compliments in order to make a sale, or in general when one person tries to flatter another in a dishonest way. Although it originally primarily referred to flattery in terms of one person attempting to gain something from another, any sort of lie can now be considered “blowing smoke” now. This includes statements made that are obviously false, such as someone complimenting an overweight person on his or her physique.
To “blow smoke” is considered a slang term and is not generally used in formal writing or speech. It is not generally included in many formal or official dictionaries, although it is listed as an idiom in others. The phrase is also no longer limited to conversations which are aimed at getting person to do something for another, but may include virtually any lie, misleading statement, and sometimes verbal irony.