Knocked down or bowled over by a nasty or disparaging comment from someone you thought you could trust, with damage to your reputation or prospects.
Here’s the image: Two or more friends are tossing light objects to one another, and you get used to catching them easily. But then your “friend” tosses you a heavy sandbag, propelled with some force. You try to catch it, but it’s way more than you expected, and it knocks you flat on your back. You feel like a fool for being knocked over and for trusting that person in the first place.
Example: I thought I was OK with him - I even thought we were friends - but then he sandbagged me with a nasty comment, in front of people who are important to me.