ABB-pattern adjective: base 阴森 ('gloomy') intensified by reduplicating 森. ABB adjectives almost always need 的 before a noun: 阴森森的房子 ('a gloomy house').
memory
森 is three 木 trees stacked = a dense forest. Triple it in this word and you get the feeling of a sunless, tree-shrouded place, exactly the mood 阴森森 conveys.
