1670s, "filled space, the fullness of matter in space" (opposite of vacuum), from Latin plenum (spatium) "full (space)," neuter of adjective plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE root *pele- (1) "to fill." Used to denote fullness in general, hence the meaning "of a full assembly of legislators" is recorded by 1772.
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