From mathach; properly, something expansive, i.e. A bag -- sack.
see HEBREW mathach
H572. amtachath
אַמְתַּ֫חַת noun feminine sack, only Genesis 42-44; — ׳א construction Genesis 44:2, 12; suffix אַמְתַּחְתִּי 42:28, אַמְתַּחְתּוֺ 42:27 2t.; plural construct אַמְתְּחֹת 44:1; suffix אַמְתְּחֹתֵינוּ 43:18 3t.; אַמְתְּחֹתֵיכֶם 43:12, 23; — sack (s) in which brethren of Joseph carried corn from Egypt; in phrase אַמ ׳בְּטִי 42:27 in the mouth of the sack, so 43:12, 21; 44:1-2, 8; elsewhere 42:28; 43:18, 21, 22, 23; 44:1, 11, 11, 12 (all J).