But, but, but...it's a phrase consisting of a verb (laughing; well, technically, "am laughing" as in "I am laughing") and adverb (out loud), usually referring to a singular person (the one typing it), so how do you make it a plural at all? Even if it was referring to a plural subject (many people), it's still "laughing out loud" not "laughing out louds" or "laughings out loud." See, this is the problem, not only is the spelling atrocious, the grammar is too!

I'm going to go back to banging my head on the desk now.