Ponder and pound respectively pertain figuratively and literally to weighing things, and this commonality isn’t a coincidence: The Latin word pondus, meaning “weight,” is the source of both words.
Ponder, meaning “consider” or “reflect” (though the original senses were “appraise” and “estimate”), stems from the Latin term ponderare (with the same meaning), the verb form of pondus. The noun form ponderance, meaning “importance” or weight,” is rare—much less common than preponderance, which refers to a superiority or majority of amount or number. The adjective imponderable, originally with the sense of “weightless,” later came to mean “unthinkable,” in the sense of something that one cannot manage to comprehend.
Meanwhile, ponderous, which first meant “thick” but later came to pertain to heaviness or clumsiness, comes from the Latin term ponderosus, meaning “of great weight” or “heavy with meaning.” Pinus ponderosa (“heavy pine”), the Latin scientific name of a type of pine tree found in the western United States, is the source of that conifer’s common name, ponderosa.
The Latin word pondus was borrowed into various Germanic languages early on; in Old English, as pund, it came to refer to a measure of weight equivalent first to twelve ounces and later to sixteen ounces. The use of the term pound for a unit of currency came about from reference to the value of a pound of silver.
The other two senses of pound are unrelated to this meaning (and to each other). Pound in reference to a place of confinement or a supply center (now most commonly pertaining to an enclosure for animals) is related to pond; both stem from an Old English verb, pyndan, which means “dam” or “enclose.” Meanwhile, the verb pound, which refers to repeated blows, is from another Old English word, punian, meaning “beat” or “crush.”