That was a real knee slapper.
Have you ever heard somebody say that after hearing a funny joke? Better yet, have you seen somebody slap their knee when hearing a funny joke?
This response to humor was probably a little more common to generations before the modern usage of LOL. Slapping the knee added emphasis to the sounds of laughter.
Knee slapping also conveyed a sense of intensity. That is, it was a response to something that was really funny. You don’t slap a knee when you only chuckle. You slap your knee when you have a hard laugh. Tears might even accompany this kind of laughter.
If you slap your knee when you are happy and are laughing, what do you do when you are sad and weeping? It seems that our Old Testament brethren slapped their thigh.
For after I had turned away, I relented,
and after I was instructed, I struck my thigh;
I was ashamed, and I was confounded,
because I bore the disgrace of my youth.’
(Jeremiah 31:19 ESV)
In this verse, Ephraim responds to the discipline received from the Lord. Ephraim had sinned against God. God disciplined Ephraim. Ephraim turned from their sin filled with shame and disgrace. This sense of shame and disgrace was evidenced by striking the thigh.
Later the prophet Ezekiel uses similar language to describe the lament over the coming discipline of Israel.
Strike therefore upon your thigh.
(Ezekiel 21:12 ESV)
The inner sense of lament or shame of sin that is expressed through the tears of sorrow can be further demonstrated by striking the thigh.
We would much rather be in a knee-slapping mood. Perhaps a bit more often we should be in a thigh-slapping mood.