despise
謾 centers on contemptuous speech: to despise, scorn, or speak deceitfully. The given meanings of despising and deceiving both stem from using words to belittle or mislead.
謾 combines 言 (speech) with 曼, which likely contributes the sound and may carry a sense of extended or excessive, suggesting speech that is overbearing or deceitful.
Words 言 that stretch out like the 曼 component (which visually suggests something drawn out) become contemptuous or deceitful speech. Imagine someone dragging out insults to despise another.
For マン, picture a man despising someone with his words: man -> マン, and his speech is full of scorn.