--a cyclist that does not wear a helmet. The bonehead part of the cyclist refers to:

a) the condition the cyclist will achieved if he/she were to crash and hit his/her head on the road, tearing up some valuable tissue and exposing bone from the skull;

b) the fact that the cyclist has nothing in their cranium but bone;

c) the use of bone to protect the head instead of a helmet;

d) behaviour on a bicycle that exudes idiocy.