Pulling or pushing with great effort; or filtering liquid through a fine mesh or cloth.
From Old French 'estreindre' meaning 'to bind tightly,' from Latin 'stringere' (to pull tight). The '-ing' makes it a present participle. The sense of filtering comes from the idea of separating things through tension.
When a muscle 'strains,' the same Latin root 'stringere' is at work—your muscle fibers are being 'bound tightly' under too much tension. The metaphor is perfect: both straining and strains involve something being pulled beyond its comfortable limits.
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