Brieriest is the superlative form of brier, which means a thorny bush or shrub. The spelling follows the general rule for adding -iest to words with two or more syllables, which is to add -iest after the word is modified with the word "most." In IPA phonetic transcription, brieriest is pronounced as /ˈbraɪər.i.əst/, with stress on the second syllable. The "ier" is pronounced as /aɪ.ər/ and the ending "-iest" is pronounced as /i.əst/.
The word "brieriest" is derived from the noun "brier", which refers to a type of thorny shrub or plant. The suffix "-iest" is added to the word "brier" to form the superlative adjective "brieriest", indicating the highest degree of having or possessing the qualities of a brier.