A radial crack or fissure in the central heartwood of a tree, caused by uneven drying or stress; also an emotional shock or upheaval.
From 'heart' plus 'shake,' a wood-working and forestry term. In timber, heartshake is a literal structural problem in the tree's heart, while metaphorically it describes emotional disturbance.
Foresters and carpenters use the word 'heartshake' to describe literally shattered tree hearts (stress cracks), and this technical term got borrowed to describe emotional shaking, showing how we use physical/material metaphors for all kinds of human experiences.
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