The Hebrew verb mush (מוּשׁ) means to depart, move away, or be removed. It often appears in negative contexts — things that should not be removed or that are promised never to depart. The word carries the idea of physical departure or withdrawal of presence. It appears in powerful covenant promises where God swears that certain things will never mush — never move or be taken away.
The most theologically significant use of mush appears in God's covenant promises where He swears that His faithfulness will never depart. Isaiah 54:10 is one of Scripture's most profound declarations: "For the mountains may depart [mush] and the hills be removed, but my steadfast love shall not depart [mush] from you, and my covenant of peace shall not be removed." God uses the most permanent things in nature — mountains — as the lower bound of His faithfulness, then declares His love exceeds even that permanence. Joshua 1:8 commands that the Torah must not depart (mush) from one's mouth — the Word of God is to be a constant, abiding presence.