From shalash; a descendant of the third degree, i.e. Great grandchild -- third (generation).
see HEBREW shalash
H8029. shillesh
II. [שִׁלֵּשׁ] adjective pertaining to the third; — only masculine plural as substantive שִׁלֵּשִׁים those of third Generation (grandsons); שׁ ׳בְּנֵי Genesis 50:23 (E) sons of those of third Generation, i.e. great-grandsons (see KöSK 1898, 533 ff.); עַלשֿׁ ׳עַלבָּֿנִים וְעַלרִֿבֵּעִים Exodus 20:5 (E; see [ רִבֵּעַ]} = Deuteronomy 5:9; Numbers 14:18, compare Exodus 34:7 (both J).