early 15c., "closely compacted, thick," from Latin densus "thick, crowded; cloudy," which is of uncertain etymology, perhaps related to Greek dasys "hairy, shaggy; thick with leaves," as a grammatical term, "aspirated," but even this is in doubt. Figurative sense of "difficult to penetrate" (of writing, etc.) is from 1732; that of "stupid" is first recorded 1822. Related: Densely; denseness.
