Why was "tantrum" trending last week?
1640s, noun of action, formed on model of progression, from Latin retrogressus, past participle of retrogradi "move backward" (see retrograde).
retrogression ret·ro·gres·sion (rět'rə-grěsh'ən)n.
The act or process of deteriorating or declining.
A return to a less complex biological state or stage.