pondus

Pondus, ponderis is third declension neuter, from pendo, "weigh out". As with corpus and tempus, the nominative tells you nothing about the stem: you simply have to know that it is ponder-, and the pattern is worth learning once for the whole set.

Beyond physical weight it means importance or authority, so homo magni ponderis is a man whose opinion counts, and the plural pondera can mean the official weights kept for trade. From it come ponder, ponderous and preponderance, and, by a longer road, English pound: the unit is the Roman libra pondo, "a pound by weight", with the noun swallowing the phrase.