The meaning of 'blow me down' [duplicate]

What's the meaning of this sentence below from Longman Dictionary of Contemporary English?

▪ Blow me down if she didn’t just run off!

http://www.ldoceonline.com/dictionary/blow-blow-me-blow-it-etc

The Longman Dictionary says you use 'blow me' to show annoyance or surprise and is usually used in British English. But I still cannot figure out what it means and in what situation it can be used.

Solution 1:

It is a term like many others whose usage has metamorphosed. Begin with "When she just ran off, you could have knocked me over with a feather!" It suggests that sudden shock and surprise renders a person weak and liable to faint. Now "She's run off ? - well blow me down!". Modify that slightly "Blow me down if she just didn't just run off". It is idiomatic.

For a wider treatment on the use of blow as an imprecation etc see the answer I have provided with OED reference at: english.stackexchange.com/q/363308/168678