Brainsick is a compound of brain + sick, from Old English brægenseoc ('mad'), literally 'sick in the brain'. Brain is from Old English brægen; sick from Old English seoc. The historical trail runs through brægen in Old English and seoc in Old English. Morphologically, brainsick is transparent: it is a compound of brain + sick, from Old English brægenseoc ('mad'), literally 'sick in the brain'. Used as an adjective, brainsick now means "mentally disordered; mad or deranged", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in derived forms such as brainsickness and related forms such as brain and sick. The modern word remains analyzable through its parts, which makes the semantic connection unusually direct.