A kingpin lock is an anti-theft device that fits over the kingpin — the pin a tractor’s fifth wheel latches onto — of an unattended trailer. With the lock in place, no tractor can couple to the trailer, so it can’t be driven off.
It’s used whenever a loaded or empty trailer is left parked and unhooked for a while. Along with a numbered seal on the doors, it’s a common way to protect trailers and their freight from theft while sitting in a yard or lot.