Although fast kink oscillations of coronal loops have been extensively studied for decades, details about their excitation, evolution, and damping remain elusive. Here, we present, for the first time, a longitudinally asymmetrical fast kink oscillation of a coronal loop that is excited by a GOES C-class flare located diagonally below the loop system. In this case, a damping-like phase and a growing-like phase are successively observed after the flare in different loop segments that are far from and close to the flare site, respectively. The investigation provides us with a new and valuable clue for the widely observed distinct deviations of fast kink oscillations from the sinusoidal pattern, especially for the local damping-like or growing-like phases associated with diagonally placed excitation sources, by illuminating a probable effect of the diagonally placed excitation source.