Closed tightly so nothing can get in or out, or to close something in a way that prevents entry or escape.
From Old English 'sielan,' possibly related to Old Norse 'sila' meaning 'to strain or sift.' The word evolved from the idea of closing or fastening something shut, with the image of pressing something down firmly.
When you 'seal' something, you're not just closing it—you're making it absolutely airtight and watertight, which is why the word 'seal' also means the animal: both seals (animals) and sealed containers keep water out with their surfaces.
Complete word intelligence in one call. Free tier — 50 lookups/day.