IPA: /ˈmɔːrnfələr/
KK: /ˈmɔrnfəlɚ/
Feeling or expressing deep sadness or sorrow, especially in a way that is more intense than usual.
The mournfuller tone of the music made everyone feel the sadness of the story.
Comparative: more mournful
Superlative: most mournful
Mournfuller → It is formed from "mourn" (from Old English *murnan*, meaning to feel or express sorrow) and "-full" (meaning full of) and "-er" (indicating a comparative degree). The word "mournfuller" means more full of sorrow or grief than something else.
Think of feeling sorrow ('mourn') and being 'full of' that sorrow ('-full'), and then consider that 'mournfuller' means having even more sorrow than before.
No commonly confused words.