Under the knife
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
Used to describe undergoing surgery but can be a metaphor for a major change or overhaul. 
The software company is going under the knife with a complete redesign of its platform.
Pro Tip 2/3Informal Tone
Common in casual conversation, it's less formal than saying 'having surgery'. 
I'm a bit nervous about going under the knife next week.
Pro Tip 3/3Negative Connotation
Often implies apprehension or reluctance about the surgery. 
After much hesitation, she decided to go under the knife to improve her health.