From πλήθω (plḗthō, “I fill”), the intransitive counterpart of πίμπλημι (pímplēmi).
πλῆθος • (plêthos) (genitive πλήθεος or πλήθους); n, third declension;
- a large number of things, a great number
- majority, main portion
- something of large size or magnitude
- specifically: a large group of people: crowd, multitude, throng, mob
- populace, people, general public
This inflection pattern originally had a sigma (i.e. *πλήθεσος), which was dropped early on in the history of Ancient Greek.
The irregular accentuation of this paradigm is explained by the fact that it is the contracted version of the above paradigm.