To be extremely familiar with something; to know it thoroughly and intimately.
This phrase emerged in the 18th century, referencing how well we know our own hands - something we see constantly and use daily. The back of the hand was chosen specifically because it's always visible to us, unlike the palm, making it a perfect metaphor for intimate, constant familiarity.
The phrase is beautifully ironic because most people actually can't describe the back of their hands in detail despite seeing them constantly - we know them through muscle memory and spatial awareness rather than visual cataloging. This reveals how 'knowing' something deeply often transcends conscious observation, involving embodied, intuitive understanding.
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