Overcome with surprise or astonishment. Also put as throw for a loop or knock sideways. The variant knock down with a feather (often put as you could have knocked me down with a feather) implies being stunned by something trivial enough to topple one.
Examples:
- The news of his death knocked me for a loop.
Origin: First half of 1900s; knock down with a feather dates from early 1800s; knock sideways first recorded 1925
Synonyms: throw for a loop, knock sideways, knock down with a feather