Expression used by cyclists to describe excercise induced low blood sugar levels; being a feeling of light-headedness and weakness in all limbs. Similar to 'The Wall' in running. Has fallen out of usage in recent years due to alternative meanings.
I am feeling a bit light-headed, if I don't have a banana, I think I'm going to bonk.
I've got the bonk.
I bonked really badly and had to get a lift home.