The compound "three-fifty" is a modern English formation, directly combining two cardinal numbers from distinct ancient lineages. "Three" descends from the Proto-Indo-European root trei-, which gave rise to the number across many Indo-European languages, signifying a quantity of one more than two. "Fifty" originates from the Germanic compounding tradition, where fīf (five) was combined with the suffix -tig, denoting a decade or group of ten. When joined by a hyphen, these morphemes create a fixed expression whose specific meaning—whether referring to a firearm caliber or a monetary amount—is entirely dependent on modern contextual usage, showcasing how numerical compounds can acquire specialized slang or technical senses.

Structure: three (number) + fifty (number) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as the cardinal number 3. - fifty: From Old English fīftig, from fīf (five) + -tig (group of ten); of Germanic origin. Functions as the cardinal number 50.