Rising or flowing upward like water from a spring; can also describe emotions that emerge and build up inside you.
From Old English 'weallan' or 'willan,' meaning 'to boil' or 'to surge.' The original sense was of water bubbling up from underground sources, later extended metaphorically to emotions.
Tears 'well up' because of actual physiology—when you're emotional, your lacrimal glands produce tears, and gravity combined with the structure of your tear ducts naturally makes them pool and overflow from the inner corner of your eye.
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