"Inkhorn words" are words that writers "invented" and introduced into the English language, usually based on Latin or Greek words.  An inkhorn, literally, was an ink bottle made out of an animal's horn; an "inkhorn" word was a word that did not originate in normal speech, but rather seemed to spring directly out of the writer's ink bottle. It was a word that, at least originally, existed in writing only.

In the 16th and 17th centuries, there was a disagreement about the use of inkhorn words. Some writers thought that Latin and Greek were more elegant than spoken English, and they generously sprinkled their writing with inkhorn words.  Here are some examples (from www.worldwidewords.org):

• eximious: excellent, distinguished, eminent.
• fatigate: to fatigue
• illecebrous: alluring, enticing, attractive.
• ingent: immense, very great.
• obtestate: to bear witness, call upon as witness

Other writers felt that such words were not a natural part of the English language. A writer named John Cheke expressed this opinion:

I am of this opinion that our own tung should be written cleane and pure, unmixt and unmangeled with borowing of other tunges.

Notice that Cheke uses the word tongue (he spells it tung), which is of Old English origin, rather than language, which is of Latin origin.