pimples
瘩 centers on skin eruptions, specifically pimples or similar raised swellings.
瘩 combines 疒 (sickness) with 荅, which likely contributes the sound. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around 荅, like a breakout of pimples trapped under the skin.
For トウ, imagine touching a painful pimple and saying 'ow!' — the 'tow' sound cues トウ.