In a way that is clearly obvious or easy to see.
From the adjective 'patent' which comes from Latin 'patens' meaning open or exposed. The 'patent' in 'patent office' is related—it's called a patent because it's an open, public claim of invention.
There's a neat linguistic coincidence: 'patently' (obviously) and 'patent' (the legal document) come from the same root meaning 'open'—patents are called that because they expose inventions publicly rather than keeping them secret.
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