large rope, rope attached to bier
紼 centers on a large, strong rope, especially one used in funerary contexts to pull a bier or coffin.
紼 combines 糸 (thread) with 弗, which likely contributes the sound. The character originally referred to a thick rope or cord, especially one used in funeral rites.
The thread radical 糸 on the left suggests a rope, while the right side 弗 looks like a twisted or braided shape. Imagine a massive, braided rope used to pull a heavy bier.
For フツ, picture a huge rope snapping with a loud 'foots' sound as it strains under the weight of the bier: foots -> フツ.