/king"pin'/ , n.
1. Bowling.
a. headpin.
b. the pin at the center; the number five pin.
2. Informal. the person of chief importance in a corporation, movement, undertaking, etc.
3. Informal. the chief element of any system, plan, or the like.
4. a kingbolt.
5. either of the pins that are a part of the mechanism for turning the front wheels in some automotive steering systems.
[ 1795-1805; KING + PIN ]