Abarim Publications' online Biblical Hebrew Dictionary
There are three separate roots of the form אבל ('bl) in the Bible, and one adverb, and they don't seem to have a lot to do with each other:
The root-verb אבל ('abel) means to mourn or lament. It's used in the expectable way of mourning for the deceased (Genesis 37:34, Isaiah 19:8, Amos 8:8) but also in a figurative sense, such as a mourning land (Isaiah 24:4) or a lamenting wall (Lamentations 2:8).
Its derivatives are:
- The masculine noun אבל ('ebel), meaning a mourning (Genesis 50:10, Isaiah 60:20).
- The adjective אבל ('abel), meaning mourning (Esther 6:12, Job 29:25).
The root-verb אבל ('bl II) isn't used in the Bible so we don't know what it may have meant. Long it was supposed that it was related to similar roots in cognate languages that meant things like to be green or grow green, but (according to HAW Theological Wordbook of the Old Testament) nowadays scholars believe that it is an "Aramaic type of participial form" of the verb יבל (yabal), denoting a being dragged or carried along by a greater force.
The derivatives of יבל (yabal) have to do with water streams or produce that comes forth from the ground. And so it is assumed that our verb אבל ('bl) has that same meaning. The only Biblical occurrence of our verb is in the noun אבל ('abel), which is spelled and pronounced the same as the adjective discussed above. But this noun occurs only as name or name-element. Older interpretations and commentaries will translate this name(-element) with meadow (after the presumed meaning of being green) but younger ones will read stream or brook.
The root אבל ('bl III) is also not used as verb in the Bible, and its sole derivative is again a name: Obil. BDB Theological Dictionary declares that this root is comparable to an Arabic verb that means to be able to manage camels.
There's one more word of the form אבל ('bl) in the Bible, and that is the adverb אבל ('abal). In the Bible's older texts, this sporadically used word appears to have an asseverative force to it; it expresses a solemn affirmation: verily or truly (Genesis 42:21, 2 Samuel 14:5). In later texts of the Bible, however, this adverb appears to express light negations (2 Chronicles 1:4, Ezra 10:13). HAW Theological Wordbook of the Old Testament proposes that our adverb אבל ('abal) is comparable to the adverb בל (bal).