To understand something deeply or completely, especially something complex or mysterious. It can also mean to measure the depth of water.
From Old English 'fæthm,' originally a unit of measurement based on outstretched arms (about 6 feet). The nautical sense of measuring water depth led to the metaphorical meaning of measuring or understanding the depth of ideas.
Think of reaching your arms wide to 'fathom' the depth - you're trying to get your arms around a complex idea! This word often appears with negative constructions ('cannot fathom').
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