kingpin

1. (transport) The axis around which steered wheels pivot; a bolt that holds the axis in place—a kingbolt.

2. (transport) The pivot or connector between a tractor and trailer in the fifth-wheel coupling of semi-trailer truck.

3. (informal) The most important person in a usually evil undertaking or organization, often a druglord.Examples: "The rare salarian females are cloistered on their worlds out of tradition and respect. Powerful female Dalatrasses are dynasts and political kingpins. They determine the political course of their respective regions through shrewd negotiation."Synonyms: big cheese, big enchilada, big tamale, big kahuna, bigwig, big wheel, grand poobah, head honchoinformal

4. (ten-pin bowling) The pin at the centre of the triangle of bowling pins (originally the tallest pin in kayles). Sometimes also the headpin at the apex.