This is a Latin expression which is used in linguistics to refer to the meaning of a word in common usage of the time. It’s often opposed to that method of determining word meaning by studying a word’s etymology. Fairbairn references this concept here. You can also see this concept referenced in Baxter:
But, as they must confess, the Creed was not delivered by the Apostles in English, and so the word “hell’ was not in the original, so if we must stick to the Creed indeed, we must translate it truly, and you must help us to some word that is of as comprehensive a signification as ἂδης is; which, as is most largely proved by Usher and Parker, besides many more, signifieth the ‘state of the dead’ in general; or as applied to souls ‘the invisible state of separated souls;’ whereas, whatever the etymology of the word “hell” be, yet we are sure that the common use [or the latin term “usus loquendi“] (which is the master of language) hath among the vulgar appropriated it to…” Richard Baxter and William Orme, The Practical Works of the Rev. Richard Baxter, vol. 14 (London: James Duncan, 1830), 383–384.
The main point is that the etymology of a word is not a statement about its meaning but about its history; it is only as a historical statement that it can be responsibly asserted, and it is quite wrong to suppose that the etymology of a word is necessarily a guide either to its “proper” meaning in a later period or to its actual meaning in that period. Barr, The Semantics of Biblical Language, 109.
As already stated, often the etymology of a word does not help determine its meaning. Therefore we need to determine its current established usage by the writer. This practice is called usus loquendi (literally, the use by the one speaking). In other words what was the customary meaning of the word when the writer used it? How he used the word in its context often helps determine its meaning. Zuck, Basic Bible Interpretation, 103.