verb

• (informal) To strike or collide with something.
• (informal, chiefly UK, ambitransitive) To have sexual intercourse (with).
  "Sophie (Katrin Cartlidge) What is a proper relationship? Louise (Lesley Sharp): Living with someone who talks to you after they've bonked ya."
• (skateboarding, snowboarding) To hit something with the front of the board, especially in midair.
• (informal, sports) To experience sudden and severe fatigue in an endurance sports event due to glycogen depletion.
  "I had eaten five of my six PowerBars. I was exhausted and famished. In cycling they describe what was happening to me as bonking: my body was out of fuel and had no more energy."

noun

• (informal, countable) A bump on the head.
• (informal, countable) Any minor collision or blow.
• (informal, countable, chiefly UK) An act of sexual intercourse.
  "“[…] It’s not like I’m just looking for a bonk, is it? This is something a bit different.” ¶ “Quite,” said Nick—though bonk was a troublingly casual way of referring to something which preoccupied him so much."
• (informal, uncountable) A condition of sudden, severe fatigue in an endurance sports event caused by glycogen depletion.