To be the foundation or basis for something; to exist as a fundamental cause or principle beneath the surface of what is observed or apparent.
From Old English 'underlicgan' meaning 'to lie beneath,' combining 'under' and 'lie.' The metaphorical sense of being a hidden foundation developed in the 16th century, moving from physical to abstract relationships.
What underlies something is often more important than what's visible on the surface - like how tectonic plates underlie earthquakes, or how childhood experiences underlie adult behavior, showing that the most powerful forces are often invisible!
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