Definition
The compound brain-wood is an interestingly specific version of the broader term wod. As Josh Eyler's analysis of this term makes clear, wod evokes both extreme mental disorder and disruptive physical behavior. Brain-wood refines this connection between mind and body by specifying the brain as the physical source or site of the illness.See its definition in the Middle English Dictionary, which makes a distinction between brain-wood and the closely related term brain-sick (with the former term conveying a more intense state of upheaval than the latter).

The fuller story
The earliest usage recorded in the Oxford English Dictionary is //William of Palerne//, a Middle English translation of a late 12th-century Old French romance. The widely disseminated poem //The Prick of Conscience// (ca. approximately 1350) provides a roughly contemporary example. As part of its presentation of hell-pains, the Conscience-poet notes that extreme hunger will cause the damned to resemble those who have become mad: "For hungre thai sal be als the brayne-wode/Bot the dede sal be thair fode." This passage does not establish a causal link between the tortures of hell and illness in the brain, but creates a complex association between the bodily suffering experienced by the damned and the humiliation and debasement inherent in losing one's rationality.

Further reading

Useful links