whitewash
堊 centers on the act of whitewashing: applying a white coating to a surface, especially a wall. The meaning is specific to this coating process.
堊 combines 亞 (a traditional form associated with covering or secondary) with 土 (earth/clay), suggesting a clay-based coating applied to surfaces. The exact historical development is uncertain.
The top 亞 looks like a complex covering, and the bottom 土 is earth. Picture a thick white clay mixture being spread over a wall to whitewash it.
For アク, imagine a worker whitewashing a wall and muttering 'ack!' when some whitewash drips on their head: ack -> アク.