Meaning
What does number 15 burger king foot lettuce mean?
The phrase refers to a picture of a Burger King employee taken while he was standing in two containers of cut lettuce ready to be served, in shoes.
The person has posted this to the messaging board called 4chan, where users made quick work of the information and found out where the picture was taken, so that they could send it to multiple local news agencies and report it to the burger king headquarters.
Origin
What's the origin of number 15 burger king foot lettuce?
Originates from the messaging board 4chan, where the picture was first posted to on 16th July 2012, with the caption: “This is the lettuce you eat at Burger King.”
Spread & Usage
How did number 15 burger king foot lettuce spread?
After its original appearance in 2012 on the messaging board 4chan, the thread and most importantly the original picture was mentioned repeatedly for weeks and the story was covered on national news.
The peak popularity of the phrase however was in late 2017 after a YouTube compilation was created, with the picture and story mentioned in it, which has spawned an entry of the whole text of the video written out and posted to /r/copypasta which is a subpage on the messaging board website Reddit.

External resources
- Know Your Meme โ Burger King Foot Lettuce