Becomes deeper or more profound; or causes something to go deeper or become more intense.
From Old English 'deop' (deep) with the causative suffix '-en.' The figurative sense of 'deepening understanding' emerged by the 1600s.
Interestingly, emotions and relationships 'deepen' but trendy fashions become 'shallow'—we use depth as a metaphor for value, suggesting profundity is what lasts.
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