senioryoudaoicibaDictYouDict[senior 词源字典]
senior: [14] Senior was borrowed direct from Latin senior, which was the comparative form of senex ‘old’. This in turn was descended from the Indo-European base *sen-, which also produced Welsh hen, Gothic sineigs, Lithuanian senas, and Armenian hin, all meaning ‘old’. English is also indebted to senex for senate and senile [17], while senior has contributed sir and the whole range of Romance terms of address, including monsieur, sennor, and signor.
=> senate, senior, sir[senior etymology, senior origin, 英语词源]
senior (adj.)youdaoicibaDictYouDict
late 13c., from Latin senior "older," comparative of senex (genitive senis) "old," from PIE root *sen- "old" (see senile). Original use in English was as an addition to a personal name indicating "the father" when father and son had the same name; meaning "higher in rank, longer in service" first recorded 1510s.

The Latin word yielded titles of respect in many languages, such as French sire, Spanish señor, Portuguese senhor, Italian signor. Senior citizen first recorded 1938, American English.
senior (n.)youdaoicibaDictYouDict
mid-14c., "person of authority;" late 14c., "person older than another," from senior (adj.). Sense of "fourth-year student" is from 1741, from earlier general sense of "advanced student" (1610s).