It is related to the word shadow
in verse Judges 9:36. Thus, Mount Zalmon
is Shadowy/Dark Mountain.
It is not to be confused with the Zalmon of Psalm 68:14, which is in the region of Bashan, northeast of the Sea of Galilee. The mountain in view in the present passage must be close to Shechem. But since no mountain by that name is otherwise attested in the area, it is likely that what we are dealing with is an alternative name for either Mount Gerizim or Mount Ebal, the two mountains near Shechem where the covenant blessings and curses had been pronounced in the time of Joshua (Deuteronomy 27:12–13; Joshua 8:33).1
48 And Abimelech went up to Mount Zalmon, he and all the people who were with him. And Abimelech took an axe in his hand and cut down a bundle of brushwood and took it up and laid it on his shoulder. And he said to the men who were with him, “What you have seen me do, hurry and do as I have done.”