The Modern Hebrew word for profession – מִקְצוֹע (meek-TSOH-ah) appears in the Bible meaning the side of a room/ a geometrical figure. The root is ק.צ.ע (k.ts.a), a root that shares its first two letters with other roots of a similar meaning: ק.צ.ה (k.ts.h) – edge; ק.צ.ר (k.ts.r) – short (in length); etc.
What does a side have to do with a profession? By gaining a profession, a person acquires his/her side or corner in the workforce. Think carving a niche.
To describe someone as professional you’d use מִקְצוֹעִי/ת (meek-tsoh-EE/EET – masc./fem.). For example, הוּא אִינְסְטֶלַטוֹר מִקְצוֹעִי (hoo een-steh-LAH-tohr meek-tsoh-EE) – he is a professional plumber.
By contrast, to say someone is a professional, you’d use מִקְצוֹעָן (meek-tsoh-AHN – masc.) or מִקְצוֹעָנִית (meek-tsoh-ah-NEET – fem.).