Well me and my older brother came up with mines, and before you ask if you enter my nickname in to goggle you'll be surprised on what you'll see since I praticly use my user name for mass majority of things
Could also say that I played around with the following words:
Me as in I or also used in Jamaca, also the same way as in ma in Glasgow when you greet someone
Man as in what I am, or also used in a Glaswegian term
And finally the 887 is just a random number I came up with because back when I started using the internet in 1999, there were already people using the same user name so added 887 to the end.