This is a line from the song 「結婚しようよ」by 吉田{よしだ}拓郎{たくろう} (youtube link)
Relevant piece of lyrics:
雨が上がって雲のきれ間に
お陽様さんが見えたら
ひざっこぞうをたたいてみるよ
結婚しようよMMMM
When I googled it, I came across 膝を叩く which seems to be used when you "get" something (an element of surprise where you tap your knees to show it) but I don't think it has that meaning here.
Also, I could not find a proper meaning for ひざっこぞう but I could find a meaning for ひざこぞう (kneecap) which means the っ is probably used for emphasis.