While looking up "pleonasm" I noticed "plenum". Compare
definitions 1 and 2. Can it be that a politcal meeting may be called a "plenum" because it is full of hot air?

plenum ['pli¢°nəm]
noun
(plural: -nums, -na [-nə])
1 an enclosure containing gas at a higher pressure than the surrounding environment

2 a fully attended meeting or assembly, esp. of a legislative body

3 (esp. in the philosophy of the Stoics) space regarded as filled with matter
Compare: vacuum [1]

4 the condition or quality of being full
[ETYMOLOGY: 17th Century: from Latin: space filled by matter, from plenus full]