scald, heat, wash, iron clothes
燙 centers on applying intense heat to a surface or object: scalding with hot liquid, heating, washing with hot water, or ironing clothes. The common thread is using heat to treat or clean something.
燙 combines 湯 (hot water) with 火 (fire), reinforcing the idea of heating or scalding. The exact historical development is uncertain, but the component combination clearly expresses heat-related actions.
Hot water 湯 sits over fire 火, like a pot of boiling water used to scald, wash, or heat clothes for ironing.
For トウ, imagine a toe accidentally touching the scalding pot: toe -> トウ, and the heat makes you pull back quickly.