Plummet?
Pendulum?
Bola is three (or more) weights tied together.
I could imagine the earliest idea of "sling(ing)" comes from a simple / primitiv transport system to transport middel weighted stones or packages of lime, sand or earth from a pit to a "building site" (over a river or some other kinds of obstacles).
But how, when or where these "idea" really come from is not really to answer, because even some kind of spiders and other animals / insects "knows" and uses the same "thing" allready. So the sling is not only 50.000 but 500 billion years old, and men are the very last who found it - or in other words: "Sling" has no predecessor - even more primitive as "sling" isn't imaginable or possible.
Otherwise it were only a string, strand, strip, rope, cord OR a simple STONE - but "sling" is the predecessor of "sheperd sling"
.
Nominally it is related to "slaughter", "slang", "lie", "layer", "lingerie" (compare: "strips" & "strip tease"), "luck", (germ.: "Schlauch"), "lock", "loch", "lake", "leg", "link", "length", "laughter", "lash" and "clock" (means in origin: "pendulum" as it is in a "metronom" too) and like "col-league" a so called "co-" or "con-extension" to the old greece verbum "legein" / "leggein" (logos, locos, locus, lingua, language, length, leggins, light, lung, ...).
So "s-ling" means right not a "sling for slinging stones" but a tool to wordly "link" (tie / bound) somthing. It's coming from gr. / lat.: "legein" / "lego" (like "league", "legion", "legislative", "lexikon", "logic", "logos", "lock" and "re-ligion").
Interessant für die deutsche Sprache:
Die Lautwurzeln zu "Sling", "Slang" und "Schlange" sind koptischer und griechischer Herkunft und (daher) ebenso mit "Klang", "Klage", "Lüge", "klug", "Glocke" (Glas / Gleis), "Schluck", "Legierung" und "Gelingen" / "Glück" / "Gelächter" / "Gelenk" / "Lenker" / "Licht" / "Laut" / "Los" / "Laus" / "Lack" / "Schlag" verwandt. Manche S~, Sch~, Ka~ oder Ge-erweiterungen der selben Verbwurzel können aber auch aus dem älteren Präfix (gr.) "ais" (lat.: es~ / ex~) resultieren, denn S und Z stehen etymologisch alternativ zu C, CH, Aspirant (H), K und G. In vielen Fällen entsprechen ihnen aber ganz einfach die latinischen Vorsilben "co~" oder "con~" i. d. Bedeutung von "mit", "an" oder "bei" (seltener: "gegen").
Also (ital.) "s-cuse" comes from "ex-(e)-cuse" / "es-cuse" - related to (engl.) "cute", "case", "cat", "get", "cash", "cause" (caution - especially: "coitus"), "coast", "costs" and "dis-cussion" - also related to germ.: "kosen" (kotzen), "kosten" und "Kuss", respectively "kiss". So "to ex-cuse s.o." means "to let him out of liabilities, hug or obligations".