Momuna, an understudied language of Western New Guinea whose affiliation is a matter of dispute, exhibits a sort of extremely marginal gender agreement that is of interest both typologically and diachronically. While adjectives are generally uninflected, the expression for ‘old (animate)’ distinguishes masculine and feminine forms. Internal evidence as well as comparison with Mek languages (traditionally considered to be a branch of the Trans New Guinea family and spoken to the north of Momuna) both and independently suggest that the Momuna forms for ‘old (animate)’ might have decategorized from complex nouns with a shared stem that originally was a term for older kinship. Typologically, marginal gender agreement in Momuna is well in line with the trend that gender, if not cumulating with another grammatical category, is not obligatory and not marked on multiple agreement targets.