mel·lif·lu·ous /məˈliflo͞oəs/ Adjective: (of a voice or words) Sweet or musical; pleasant to hear.
As in the title: is "mellifluous" onomatopoeic or is the definition of onomatopoeia stricter than that?
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The word you are looking for is not onomatopoeic, but rather either of autological or homological, as described in this answer. Both mean a word that is self-descriptive. Mellifluous is autological because its name sounds so sweet.
A word is generally considered onomatopoeic when it has its origins in the actual sound it describes. Common examples are buzz, whoosh and cuckoo. Melliflouous comes from Latin words meaning ‘honey’ and ‘flow’. If the sound of the word in any way echoes its sense, it does so entirely fortuitously.
According to Etymonline:
early 15c., "sweet, pleasing" (of an odor, a style of speaking or writing, etc.), from L.L. mellifluus "flowing with (or as if with) honey," from L. mel (gen. mellis) "honey" (related to Gk. meli "honey;" see Melissa) + -fluus "flowing," from fluere "to flow" (see fluent). Related: Melifluously; melifluousness.
In order for it to be onomatopoeic, at some point in its history it would have had to be imitative of some sound made. As far as I can tell, honey does not make a sound. Ever.