I watched the second half of Archer: Vice (Season 5), and across multiple episodes one ISIS employee would pelt another with a quick flurry of slaps punctuated by a final larger blow. It seems like it just started out of nowhere, so I might have missed an episode, but when did this slapstick routine start?
This slap pattern started in Season 5. I'm not sure exactly what episode, but it is definitely in Season 5. I think it is within the first 4-5 episodes.
I've watched Seasons 1-4 many times over (as they are on Netflix), and while there is slapping, the pattern of many light slaps ending with a larger slap is not present.