There’s a climb in Gran Canaria called the Valley Of The Tears. It’s remorselessly steep and it’s often very hot.
First time I tried it, my heart rate and breathing was rising alarmingly on the steep section so I stopped to let them subside, then carried on. I had to do this twice. That’s not a bonk, that’s just reaching your limit.
Second time it was 10 degrees hotter and I came to a grinding halt along a section that wasn’t even the steepest, and had to walk the bike for a while, leaning over the handlebars for support and taking frequent breaks, before finally re-gaining enough energy to carry on. THAT’S a bonk.