dirt under the nails; grime under fingernails
Refers to the dirt or grime that accumulates under fingernails. A common, everyday word.
There's dirt under my nails, so I'll trim my nails and clean them.
I cleaned the dirt from under my child's nails.
Compound of 爪 (つめ, 'nail') and 糞 (くそ, 'dirt, excrement'). The second element is used here in its extended sense of 'filth' or 'grime'.