Jamieson, Fausset and Brown on 2 Kings 14:9

9. Jehoash the king of Israel sent to Amaziah--People in the East very often express their sentiments in a parabolic form, especially when they intend to convey unwelcome truths or a contemptuous sneer. This was the design of the admonitory fable related by Joash in his reply. The thistle, a low shrub, might be chosen to represent Amaziah, a petty prince; the cedar, the powerful sovereign of Israel, and the wild beast that trampled down the thistle the overwhelming army with which Israel could desolate Judah. But, perhaps, without making so minute an application, the parable may be explained generally, as describing in a striking manner the effects of pride and ambition, towering far beyond their natural sphere, and sure to fall with a sudden and ruinous crash. The moral of the fable is contained in 2Ki 14:10.