means Fix My Lighthouse. Back in the 17th Century, lighthouse keepers were tthe butt of everyone's jokes. When a lighthouse keeper was made fun of, his retort would be "why dont you fix my lighthouse!" which was shortened over time to FML. This term was made famous in the great battle of the Sole Bay in 1902 where 14 peolpe died and 1 person broke their glasses.
"if you are going to take the piss out of me then why don't you FML!