"one who or that which condenses," 1680s, agent noun from condense. Given a wide variety of technical uses in late 18c. and 19c.
1560s, "directed downward," from down (adv.). Sense of "depressed mentally" is attested from c. 1600. Slang sense of "aware, wide awake" is attested from 1812. Computer crash sense is from 1965. Down-and-out "completely without resources" is from 1889, American English, from situation of a beaten prizefighter.