Well, on second examination, it appears to be used for masters of something, whether they be coaches or professors or persons of internet fame, and given your comment earlier, I think that's my final guess, however wrong it may be (that is, without further information, I cannot make a better guess):
Ertxiem is "Meixtre" backwards, which is a Portuguese-language nickname given to those who are masters of something (perhaps in your case, mathematics and physics?), as it sounds somewhat like "mestre".
