So if we go back in Phineas and Ferb history, there is an episode titled "Comet Kermillian." If you watched Phineas and Ferb, you would most likely remember the iconic song "Squirrels in my Pants" which was shortened to the acronym "S.I.M.P." This episode was aired on television in 2008 and the slang word "simp" really started to be used in commonly roughly mid 2019. Since "Coment Kermillian" was aired before "simp" became really popluar, we can guess that the internet slang word, "simp" is an acronym for "Squirrles in my Pants." This means that when you call someone a "simp" you are actually calling them a "squrriels in my pants."