pencil case; pen case; brush case
A container for writing or painting tools, such as pencils, pens, or calligraphy brushes. The term is neutral and commonly used in everyday contexts.
I bought a new pencil case.
筆入れに鉛筆と消しゴムを入れている。
I keep pencils and an eraser in my pencil case.
Compound of 筆 (fude, 'brush, writing tool') and 入れ (ire, 'container, case'). The exact historical derivation is straightforward.