To make something unable to move or stay completely still, like when a doctor puts a cast on a broken arm.
From French 'immobiliser,' combining Latin 'in-' (not) and 'mobilis' (moveable); the suffix '-ize' means to make or cause to be.
Before modern casts could immobilize broken bones, doctors had to keep patients completely still for weeks—if the bone shifted even slightly, the person might be permanently disabled, so 'bed rest' was literally life-changing medicine.
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