What does Ertxiem stand for?

[ArtDrake]

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".

[Ertxiem]

[spoiler=the answer]
Yup. That's about right. When I completed my MSc in Maths, my friends started to call me 'meixtre'. They said 'meixtre instead of 'mestre' because at that time there was a humor TV show where one of the characters used to say that.
[/spoiler]