To hold firmly with the hand or hands; a firm hold or grasp. Can also refer to understanding or mental grasp of something.
From Old English 'grippan' meaning 'to grasp, seize', related to Germanic roots meaning 'to grab'. The metaphorical sense of mental grasp developed from the physical meaning of holding firmly.
The phrase 'get a grip' brilliantly uses our physical experience of grasping objects to describe mental control - we understand emotional stability through the metaphor of our hands holding something securely.
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