As an adjective, it means older or higher in rank or position. As a noun, it can mean an older person, or a student in their final year of high school or college.
From Latin 'senior' meaning 'older', the comparative form of 'senex' meaning 'old man'. It entered English through French and Latin legal and social terms.
The same root that gives us 'senior' also gives us 'senate' and 'senile'—all circling around age. When you call a last-year student a 'senior', you’re jokingly treating them like the 'elder' of the school.
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