Probably my favorite inside joke is between my sister, my mom, and I. It originated from an argument my mom and dad had over my dad never wanting to get rid of old clothes. Because he never gets rid of anything, voluntarily, over the years, my mom has taken to doing his spring cleaning for him. Whenever his drawers and closets start overflowing, my mom goes through his stuff and throws out (donates) anything that's really old - like super-faded-and-holey old. And every year this starts a little argument between the two of them, that my sister and I (if we're around) sit nearby and side-eye each other about it. (They've been married for about 20 years and it shows.)
So during one argument, my mom told my dad that some of the clothes were not only old but they no longer even fit, so much so that they hung off him. She suggested that he throw it all in the garbage! to which he responded, "Why don't you throw YOURSELF in the garbage!" to which my sister and I burst out in uncontrollable laughter because it's possibly one of the more ridiculous things they've ever said to each other.
So now, whenever someone's being annoying or ridiculous or doing something that's (almost comically) frowned upon, we yell, "Just throw her in the garbage!"