The son of ChieftainArador of the Dúnedain, and a descendant through many generations of Isildur himself. Of Arathorn's early life, little is known. He first appears in annals at the age of fifty-six, seeking the hand of Gilraen as his wife. Gilraen at that time was just twenty-two years old, and though the wedding was at first opposed by her father Dírhael, she became Arathorn's wife.
The following year, Arathorn's father Arador was slain by trolls, and Arathorn took on the title of Chieftain of the Dúnedain. Gilraen bore him a son, who was given the name Aragorn. Aragorn would not know his father for long - when he was just two years old, Arathorn went hunting Orcs, and was shot through the eye by an arrow. After Arathorn's loss, Gilraen took their son to Rivendell to be fostered by Elrond.
Tolkien reveals something of the etymology of Arathorn's name in a late letter: 'The name contains an abbreviated form of þorono (thorono) 'eagle', seen in Thoron- dor, Thorongil...' (The Letters of J.R.R. Tolkien, No 347, dated 1972).