Cutting a fingernail or toenail too short — cutting it down to the quick; trimming the nail so deep it causes pain or injury.
昨日深爪してしまって、指が腫れて痛い。
I cut my nail too short yesterday and my finger is swollen and painful.
深爪はばい菌が入ることがあるので消毒した方がいい。
If you cut a nail too short, bacteria can get in, so it's best to disinfect it.
子どもが深爪をして泣いていたので絆創膏を貼った。
The child had cut their nail too short and was crying, so I put on a bandage.
The deep part of the nail (or a deeply set/embedded nail) — referring to the part cut too close or a nail embedded into the flesh.
深爪の部分が化膿して病院で診てもらった。
The area where the nail had been cut too deep got infected, so I had it checked at the clinic.
深爪による傷口の治療には消毒と軟膏が必要だ。
Treating a wound from cutting a nail too short requires disinfecting and ointment.