A bonk is a good shag. Failing that, it is a condition well known to cyclists which develops shortly after one has acquired "hunger knock". The symptoms are a raging thirst accompanied by light-headedness and sudden weakness and tremor in the arms and legs. The inability to grip the handlebars properly any more is a good sign. You have gone on for too long without adequate intake of food and drink. The only solution (which makes you feel like shite but which at least allows you, after a few minutes, to continue with your ride) is to consume a pack of glucose tablets (or a half-packet of chocolate Hobnobs) washed down with a pint of tea or Coca Cola. Bonk for a cyclist is no joke, and should be avoided at all costs. 'Tain't good for you to have your blood sugar zooming up and down in that manner.