Meshuggah (MEShUGE) is an adjective in Yiddish, or, to be more specific, a predicate adjective (similar to English afraid, alive, alone). It is not a noun at all. The noun forms -- meaning crazy person(s) (male/female/plural) -- are as follows: MEShUGENER/MEShUGENE/MEShUGOYIM. The form when used as an attributive adjective, i.e., an adjective that modifies a noun, is MEShuGEN. Example:
der khosn iz meshuge. di kale iz meshuge. a meshugener khosn. a meshugene kale. A meshugener. A meshugene. Tsvey meshugoyim.
(The bridegroom is crazy. The bride is crazy. A crazy bridegroom. A crazy bride. A crazy man. A crazy woman. Two crazy people.)