look up any word, like lemonparty:
 
1.
mrx
Short for merci, in the same way that thx is short for thanks, and grx is short for gracias.
TonAmi588: bonjour, whatsup?

PasProb83: hey :) what can I do for u?

TonAmi588: I actually just wanted to ask if u still had ur key, I left mine at home

PasProb83: O yeah come by & u can have it, belle soeur

TonAmi588: mrx!!!

PasProb83: yeah sis no problem
by JustSayin :) July 26, 2011
 
2.
Mr. X is the male version of the letter "X" in the English alphabet. Mrs. or Ms. X is the female version. Since they have no genitalia it is hard to differentiate between the two.
abcdefghijklmnopqrstuvwxy and z - Mr X is right in there towards the end.
by Junkrocka May 15, 2008
 
3.
mister X is a person that you talk about with friends in code, usually a dealer of things that are code-worthy. See Mr. Nice Guy.
Are you going to mr x's? To get that stuff? Cool.
by mogrove42 July 21, 2006