Coney-Grelvish is a general term for fangirl Lapine. As its name implies, it's a variant on the phenomenon of Grelvish that applies to the language of the rabbits in the Watership Down fandom.
Lapine[]
The rabbit characters in Watership Down communicate in Lapine. Rather than a purely spoken language, it is composed of vocalisations, body language, pausing between ideas, and foot thumping or scratching. Due to the written and on-screen media in which WD is presented, it usually appears to the audience as plain English, with some Lapine words interjected for character names, human concepts the rabbits made their own names for, and phrases related to animals life that don't have any human translation.
Aside from character names, we mostly only have translations for times of day ('silflay' is 'outside feeding', 'fu inlé' is 'after moonrise', 'ni-frith' is 'noon') and a few common nouns ('homba' is 'fox', 'thlay' is 'fur', 'hrududu' is 'one of those big noisy things that humans move around in'). We also know rabbits can't count higher than four, with any number higher just designated 'hrair' for 'many'. (The rabbit named 'Hrairoo', 'Little Thousand', has his name consistently translated to 'Fiver' in English, as he was the fifth rabbit born in his litter.) Nouns are made plural by dropping any ending vowels and adding the '-il' suffix to the end. All officially existing Lapine words appear in a glossary at the end of WD, reproduced here.
Lapine in Fanfiction[]
Because Lapine is far from being as complete as the languages J.R.R. Tolkien created for Middle-earth, it can be difficult for a fan author to be very creative with it without creating new words and phrases. Original Lapine use in fics may or may not constitute a charge in a mission, depending on how well it fits in with Richard Adams's original work. If it clearly contradicts the established language and rules, it's Coney-Grelvish.