Q: I studied Latin in school decades ago, but I don’t remember the prefix “ob-.” It came up in connection with the word “oblong.” In searching online, “ob-” has a lot of meanings, as is usual with Latin prefixes. Can you clarify how it’s used in referring to an oblong shape?
A: The word “oblong” comes from oblongus, classical Latin for elongated. It combines the prefix ob-, which has a couple of possible meanings here, and the adjective longus, or long. An ancient Roman would have used oblongus to describe something that’s greater in length than in width.
The Chambers Dictionary of Etymology says the ob- in oblongus is being used “perhaps in the sense of to or toward but also functioning as an intensive.” However, the Oxford English Dictionary says, “The exact force of the prefix in oblongus is unclear: there is no analogous word in Latin.”
As the OED explains, oblongus is an oddball in Latin, where ob- usually combines with verbs and their derivatives. It says the prefix “was rarely combined with an adjective (the chief example being oblongus).”
The prefix is also easy to miss, since its form can change to match the first letter of a combining word. It’s oc- before verbs and derivatives with c as the first letter, of- before f, and op- before p.
The prefix has many meanings in Latin, all of them seen in English. Here are a few: to or toward (as in oboedire, to listen to or obey); against (opponere, to oppose or be against); upon or down (obligare, to bind down); and as an intensifier (obdurare, to harden or persist).
When “oblong” appeared in English in the early 15th century, the OED says, it meant “elongated (usually as a deviation from an exact square or circular form); esp. rectangular with the adjacent sides unequal.”
The dictionary’s first citation is from an anonymous Middle English translation of Grande Chirurgie, a 14th-century medical treatise by the French surgeon Guy de Chauliac:
“Somtyme forsoþ it ocupieþ not bot o partie, and þan þingez semeþ of diuerse fourmez, lunarez, i. mone lich, fenestrate & oblonge” (“Sometimes forsooth it [a cataract] occupies not but a part, and then we see things in diverse forms—crescent-shaped, moonlike, fenestrated [with a window-like opening], & oblong”).