As an adjective, major means very important, serious, or large in size or number. As a noun in education, it is the main subject a student specializes in at college or university.
From Latin 'maior,' the comparative of 'magnus' meaning 'great,' so 'maior' meant 'greater.' The word kept the idea of something larger or more important in various uses, including military rank and academic focus.
Calling your main college subject a 'major' quietly ranks it above your 'minor' interests, as if your mind were an army with officers and foot soldiers. The language nudges you to treat some kinds of learning as more officially important than others.
Complete word intelligence in one call. Free tier — 50 lookups/day.