MOUNTAINSIDE Meaning of MOUNTAINSIDE in English moun ‧ tain ‧ side /ˈmaʊntənsaɪd, ˈmaʊntɪnsaɪd $ ˈmaʊnt ə n-/ BrE AmE noun [countable] the side of a mountain:Great rocks rolled down the mountainside. Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012