se·nile / ˈsēˌnīl; ˈsen-/ • adj. (of a person) having or showing the weaknesses or diseases of old age, esp. a loss of mental faculties: she couldn't cope with her senile husband. ∎ (of a condition) characteristic of or caused by old age: senile decay. ∎ Geol. approaching the end of a cycle of erosion.• n. a senile person: you never know where you stand with these so-called seniles.DERIVATIVES: se·nil·i·ty / siˈnilitē/ n.