Cleaning or clearing by flowing liquid through, or the reddening of skin due to emotion or heat.
From Middle English 'flusshen', possibly imitative of water sounds or related to 'flash'. The meaning of skin reddening developed from the idea of sudden rushing or flowing, like blood to the face.
Flushing demonstrates how English loves onomatopoeia - the word itself sounds like rushing water, which perfectly matches both its literal meaning of water flowing through pipes and its figurative meaning of blood rushing to create a blush. The connection between plumbing and blushing isn't accidental; both involve sudden flows through channels.
Complete word intelligence in one call. Free tier — 50 lookups/day.