Bereshit 35:18

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

וַיְהִ֞י בְּצֵ֤את נַפְשָׁהּ֙ כִּ֣י מֵ֔תָה וַתִּקְרָ֥א שְׁמ֖וֹ בֶּן־אוֹנִ֑י וְאָבִ֖יו קָֽרָא־ל֥וֹ בִנְיָמִֽין׃

And it came to pass, as her soul was departing (for she died), that she called his name Ben-oni: but his father called him Binyamin.