"Blow me down" is an English idiom that is often used when someone is surprised or stunned by a particular occurrence. The meaning of the idiom derives from the fact that the astonishment of the event is so great that the person has figuratively been knocked to the ground by it. This particular phrase gets its origins from old sailing jargon and other similar expressions that emanate from the word "blow." It is closely related to the phrase, "knock me down with a feather," which suggests that a person is so surprised that he or she can be knocked over by the lightest object imaginable.
Many English-speaking people use certain phrases that have a colorful and evocative meaning far removed from the literal meaning of the included words. These phrases are known as idioms, and they gain their meaning when people in the culture use them to describe similar sets of circumstances. Some of these idioms are used to express extreme surprise or astonishment.
Basically, individuals use this phrase when they have either witnessed or heard something out of the ordinary or different from what might be expected. The idea is that the unexpected news is so surprising that it knocks them over in the figurative sense. As an example, someone might say, "Well, blow me down, I can't believe they're getting married."
Some people might be able to guess that the origins of this phrase come from sailing circles. The centuries-old sea shanty "Blow the Man Down" has survived to modern times, and the American cartoon character Popeye, who was also a sailor, often used the phrase whenever he was put in some difficult predicament by his adversary, Bluto. Since winds were such an important part of a sailor's life, many phrases emanated from that setting that have some variation of the word "blow" involved.
There are times when people who use this idiom add a prepositional phrase at the end which includes the lightest possible object that can be imagined. In most cases, this object is a feather. For example, someone might say, "Well, you can knock me down with a feather if he shows up here today after all he's done." The meaning of this sentence is that the speaker would be totally surprised if the person to which he is referring arrives.