To opine is to express an opinion about something.

// Many people opine that social media platforms should be better regulated.

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“Entertainment will always be inherently social. This latest age has prompted society to take a page out of video game culture, which has long relied on building a digital community. We text those we care about after a cliffhanger season finale and jump on social media to opine on the latest movie or show, sharing views on the best scenes or even recording oneself watching an intense scene for posting on social media.” — Geir Skaaden, Variety, 28 Aug. 2024

We are not opining—that is, expressing our opinion—when we say that opine is not a back-formation of opinion, though the two words do share a common ancestry. A back-formation is a word formed by the subtraction of part of an existing word; for instance, the verb bartend is a back-formation of the noun bartender. Opine and opinion, however, both entered English independently, taking different routes from their mutual roots in the Latin verb opīnārī, meaning “to have in mind” or “to think.” Opinion arrived in the 14th century, while opine followed about a century later.