• 専門せんもん — specialty
• 専用せんよう — exclusive use
• 専攻せんこう — academic major
• 専念せんねん — devotion

専 likely depicts grasping or controlling, and with 寸 (hand/measure) it emphasizes doing something exclusively or with full attention. It’s productive in words about focus and specialization.

It’s a 5th-grade 教育漢字 and very common in academic and professional terms.