To break through or pass into something; to understand or figure out a mystery or difficult concept.
From Latin 'penetrare' (penetr- related to 'penes' meaning inner). Used literally for physical breakthrough and figuratively for mental understanding since Roman times.
The dual meaning of 'penetrate'—both physically breaking through AND mentally understanding—appears in many languages, which suggests our ancestors thought about understanding something as similar to breaking through a barrier, showing how physical and mental concepts intertwine in language.
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