A rejection or refusal, usually one that properly dents your confidence. You can get a knockback from a job, a loan, a landlord, or a romantic punt that went nowhere. It’s not just a “no”, it’s a “no” with a little sting, like you got politely shoulder-checked by life.
"Put in for that bar job and got a knockback. Manager goes, Cheers for coming in, mate. So I went next door, had a pint, and pretended it was my choice"