A dear friend, close family member, or someone regarded as emotionally connected and intimate like blood relatives.
Compound of 'heart' (Old English 'heorte') + '-ikin' (diminutive suffix from Middle Dutch 'kin'). Suggests someone so close that they're treated like family by blood relation.
Words like 'heartikin' and similar 'heart-' compounds show that medieval and early modern people literally used organ names to describe psychological relationships—your 'heartikin' was someone who had access to your heart in a way that transcended social hierarchy.
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