|Easton's Bible Dictionary|
Occurring only in the title of Psalm 9. Some interpret the words as meaning "on the death of Labben," some unknown person. Others render the word, "on the death of the son;" i.e., of Absalom (2 Samuel 18:33). Others again have taken the word as the name of a musical instrument, or as the name of an air to which the psalm was sung.
Int. Standard Bible Encyclopedia
muth-lab'-en (muth labben, "death of Ben," or "of the son"; Psalm 9, title).
Muthlabben (2 Occurrences)
Psalms 8:9 O Jehovah, our Lord, How excellent is thy name in all the earth! Psalm 9 For the Chief Musician; set to Muthlabben. A Psalm of David. (ASV)
Psalms 9:1 I will give thanks unto Jehovah with my whole heart; I will show forth all thy marvellous works. (See JPS BBE DBY WBS)