short answer:

Habiru was a derogative term found throughout the ancient Near East that referred to nomads, mercenaries, and people of the like. It originally did not take into account nationality or ethnicity, but eventually came to refer specifically to the Hebrews.

More detailed answer:

Habiru (Apiru being the Egyptian equivalent) was originally an appellative, referring to both Semitic and non-Semitic people. The word acquired ethnic connotations though, eventually becoming a gentilic and coming to refer specifically to the people we know today as Hebrews.

We find the word many places. In the Old Testament, the first person to be called 'ibri was Abram (Gen. 14:13). The word in that context is not an ethnic term but an appellative, and the meaning in Hebrew is "one who crossed, a transient, a nomad." The root in Hebrew is abar, which, in its transitive sense means "to cross", and in its intransitive sense, "to be a crosser, a transient, a nomad." 'ibri is strictly a gentilic in Hebrew, but with its gentilic ending removed it is found as the name of the ancestor of the Hebrews, Eber. This name manifestly traces back to the earlier form 'abir(u), whose exact cuneiform equivalent is habiru. And this word appears in cuneiform literature throughout the Near East from the twentieth century to the eleventh century.

Some examples of occurrences of the word are in texts of the Larsa dynasty, in a letter written during the time of Hammurabi discovered at Mari, in Akkadian texts found in Nuzi, in Hittite texts found in Asia Minor, and in the Tell el-Amarna Letters, found in Egypt.

habiru was always used as a term of reproach, as was its equivalent in Hebrew, 'ibri. In whatever context we find the term habiru, it always refers to aliens, soldiers of fortune, or men without a country.

While habiru is the word used in cuneiform literature, its exact equivalent in Egyptian texts is 'apiru. We find seven occurrences of apiru, and in all cases the word is written in the 'group writing', where Egyptians would list foreign words and armies.

source:

Hebrew Origins, Theophile James Meek