To shock, confuse, or astonish someone to a point of complete bewilderment; throw someone for a loop. More often phrased as "you could have knocked me down with a feather," expressing great bewilderment or surprise. It really knocked me down with a feather to hear that my ex-wife had already remarried. I was so taken aback when I found out I'd won the lottery that you could have knocked me down with a feather!