Easton's Bible Dictionary (1897)
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 Sam. 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.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
MUTH-LAB'BEN, in the title to Ps 9, is thus explained by the Rev. A.R. Fausset: "Labben is an anagram for Nabal, 'the fool' or 'wicked,' concerning the dying (ninth) of the fool." This interpretation harmonizes well with the contents of the Psalm. But the titles to the Psalms are often enigmas.