Abarim Publications' online Biblical Hebrew Dictionary
There are two versions of the same root: עמש ('amas) and עמס ('amas). Version עמס ('amas) appears to be older; it's used in the older texts of the Bible (like the Psalms). In younger texts of the Bible (like the Books of Samuel and Kings), our verb is spelled as עמש ('amas):
Both verbs עמס ('amas) and עמש ('amas) mean to load (Genesis 44:13) or carry a load (Isaiah 46:3) or burden (Isaiah 46:1).
The derived feminine noun מעמסה (ma'amasa) means load or burden (Zechariah 12:3 only).