Entish is the unifying and most common language of the Ents, a race of tree-like spirits created by the Valar Yavanna. Their language appears to be based on an ancient form of Common Eldarin, later enriched by Quenya and Sindarin, although it includes many unique 'tree-ish' additions.
Originally, the Ents had a language of their own, described as long and sonorous. Only Ents spoke Old Entish; not because they kept their language a secret, as the Dwarves did with Khuzdul, but because no other could master it. It was quite an alien language to all others. The Huorns and trees of Fangorn forest could understand Old Entish and converse with the Ents and each other with it. The only extant sample, a-lalla-lalla-rumba-kamanda-lindor-burúme, the word for hill (or possibly even just part of the name of a specific hill), was described as a very inaccurate sampling. Even the Elves, master linguists, could not learn Old Entish, nor did they attempt to record it due to its complex sound structure. The grammar structure of Old Entish was also quite bizarre, often described as a lengthy, long-winded discussion of a topic. There may not even have been a word for yes and no: such questions would be answered by a long monologue on why the Ent in question did or did not agree with the Ent who asked the question. Ents as a rule would say nothing in Old Entish unless it was worth taking a long time to say. For everyday language function, they usually resorted to "New" Entish.
Due to contact with the Elves, the Ents learned much from them. The Ents found the Elvish language Quenya to be a lovely language, and adapted it after their fashion to everyday use. However, they basically adapted Quenya vocabulary to Old Entish grammatical structure. Thus, unlike Old Entish, the individual words of "New Entish" that characters such as Treebeard spoke were easily translatable.