What does shirk mean?

Answer 1

General Definition

It depends on the context.

It generally means: to avoid or neglect (a duty or responsibility). -- i.e. He shirked his duty to provide me the documents on time. (He failed to do his duty...)

However, in the case of Islam, shirk means: to bring [other deities] into partnership to Allah. -- i.e. to blaspheme, to apostasize, to be a polytheist or atheist.

Further to the Islamic Definition

Shirk means attributing partners to Allahu ta'ala. A person who does the attributing is called a mushrik(polytheist). There are kinds of disbelief. The worst, the gravest kind is shirk. It has been a generally accepted rule that when several subjects are to be referred to under one common nomenclature, the gravest one is mentioned. For this reason, the word shirk used in Qur'anic verses and hadith-i sharifs comprehends all sorts of disbelief.

The Qur'an al-karim says (what means):

(Allahu ta'ala never forgives shirk[all sorts of disbelief]. He forgives [all] sins [venial or grave] except shirk if He wills.) [An-Nisa' 48]