early 15c., "thicken, make more dense or compact" (implied in condensed), from Old French condenser (14c.) or directly from Latin condensare "to make dense," from assimilated form of com-, here perhaps an intensive prefix (see com-), + densare "make thick," from densus "dense, thick, crowded," a word used of crowds, darkness, clouds, etc. (see dense).
Sense in chemistry and physics, "to reduce to another and denser form" (as a gas or vapor to a liquid) is from 1660s. Intransitive sense "become denser" is from 1650s. Related: Condensed; condensing.