/ ˈeldɪst; NAmE / adjective
1.
( of people, especially of three or more members of the same family ) oldest :
Tom is my eldest son.
2.
the eldest used without a noun immediately after it to show who is the oldest of three or more people :
the eldest of their three children
➡ note at old
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WORD ORIGIN
Old English ieldest , eldest , of Germanic origin; related to German ältest , also to old .