þeirra

Etymology
Genitive of the plural personal.

Pronoun

 * 1) their, of them (referring to men), the genitive plural form of hann meaning "he"
 * 2) their, of them (referring to women), the genitive plural form of hún meaning "she"
 * 3) their, of them (referring to men and women, or children), the genitive plural form of það meaning "it"

Etymology
See.