In a book called *[Intellectuals and Society, A Study of Teachers in India][1]*
by Kameshwar Choudhary (2004), there is a more thorough research about the origins of the term:

>  The term ‘intellectual’... came to the English language partly through French in which it is *Intellectuel*, the word ***used for the first time in French in 1265*** as a noun signifying *one who is concerned with knowledge or
understanding.*
>
> According to The *Oxford English Dictionary*, the term intellectual signifies
>>a person possessing or supposed to possess superior powers of intellect.
>
> It cites a number of examples of former use of the term. In **1652**, for instance, Benlowes talked of
>> First race of ***Intellectuals***. 
>
>In **1813**, Byron said, 
>> Canning is to be here, Frere and Sharpe, — perhaps Gifford... I wish I may be well enough to listen to these ***intellectuals***.
>
> In **1884**, A.A. Watts referred to 
>> The silent person who astonished Coleridge at a dinner of ***intellectuals***. 
>
> On 30 November **1898** the *Daily News* reported,
>> Proceeding to refer to the so-called ***intellectuals*** of Constantinople,
who were engaged in discussion while the Turks were taking possession of the city.
>
> On 19 December **1903**, the *Saturday Review*
stated, 
>>We are compelled to rank higher the mind of the average
young man of fashion than the mind of the average “***intellectual***”
_ at those literary tea-parties.
>
> On 12 August **1960**, the *Times Literary Supplement* mentioned,
>>The English have a great respect for brute facts; and the ***intellectual*** in politics often looks to them like a man busily engaged in brushing unpleasant facts under the carpet. 

The same author then goes on to quote [Raymond Williams][2], the Welsh writer:
> Raymond Williams holds that the word ‘intellectual’ has been
an ordinary objective, from the fourteenth century, for ‘intelligence’
in its most general sense. However, *it became a noun to signify the
faculties or processes of ‘intelligence’.* According to him, ***the use of the word ‘intellectual’ as a noun to refer to a particular kind of
person or a person doing a particular kind of work dates effectively
from the first third of the nineteenth century though it had some
isolated earlier instances***. Moreover, he notes the application of its
plural ‘intellectuals’ from the first third of the nineteenth century
to indicate ‘a category of persons,’ often unfavourably.


  [1]: https://www.google.co.uk/books/edition/Intellectuals_and_Society/kZRbyJdNBD0C?hl=en&gbpv=1
  [2]: https://en.wikipedia.org/wiki/Raymond_Williams