“Knock me over with a feather”

My roommate and I, Samantha was sitting and eating dinner one night.  She was trying to have a conversation with me but I could not seem to concentrate.  I kept yawning.  Next I began to complain about how tired I was.  My roommate responded with “knock me over with a feather”.  I had never heard this expression before.  But is makes complete sense.

After yawning repeatedly, and then proclaiming that in fact I was extremely tired, Samantha responded with a very sarcastic statement.  In truth she could never be knocked over with a feather because she is way to heavy for a feather to push.  This statement is very obvious, just as my statement, of being tired, was.