Either removing liquid from something, or exhausting someone's energy and making them very tired.
From Old English 'drehnian' and Old Norse 'drjúginn' meaning to flow. The word originally referred to water flowing away, then expanded to mean exhaustion of resources or energy.
The fact that 'draining' works for both water and emotions shows how metaphorically connected our bodies are to nature—when you're emotionally exhausted, you really do feel 'emptied out' like a bathtub after the plug is pulled!
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