Abreviation for "My face when" This is commonly used on the forums of 4chan, in addition with green text stories. Usually, the picture is on the side of all the green text, and usually at the end, "mfw". It's particulary a meme face Ex.
Forever alone, Troll face, Y u no. etc.
>Sister embarrases me in front of friend
>Don't masterbate for 2 weeks
>Start fapping in the shower
>See my sisters bodywash
>Shoot a huge load
>mfw my dog smelling my sisters ass all day