Rashi on Genesis 35:18/רש"י על בראשית ל״ה:י״ח

1בן אוני means SON OF MY SORROW (Genesis Rabbah 82:9).

2בנימין BENJAMIN — I am of opinion that he was so called because he alone was born in the land of Canaan which is in the South (ימין) as one comes from Aram-Naharaim, as it is said (Numbers 33:40) “in the South, in the Land of Canaan”, and (12:9) “going on still towards the South”. בנימין therefore means בן ימין where ימין has the same meaning as in (Psalms 89:13) “The North and the South (ימין) Thou hast created them”. For this reason the word is here written plene (with a י after the מ).

3Another explanation of בנימין is that it means “son of his old days (ימים)”, only that it is written with ן (instead of ם), like (Daniel 12:13) “at the end of days.