Atsum describes something mighty, powerful, vast, or exceedingly numerous. It appears about 31 times and is used for powerful nations, mighty armies, and the overwhelming greatness of God's works. From the root atsam (to be mighty/numerous), it often describes forces that overwhelm human capacity.
Atsum is used in Deuteronomy to describe the nations that Israel would face as 'stronger and more numerous than you' — making the conquest of Canaan depend entirely on God's power, not Israel's. This sets up a recurring theological pattern: God deliberately leads His people into situations beyond their capability so that the victory clearly belongs to Him. Isaiah uses atsum for God Himself — His mighty deeds and 'arm' of power (Isaiah 40:26). The lesson embedded in this word is that human inadequacy before overwhelming odds is exactly where divine power is revealed most clearly.