As mentioned in Mahabharata Adi Parva chapter 74 verse 39, When Shakuntala went to King Dushyanta he said this statement towards him.
पुन्नाम्नो नरकाद् यस्मात् पितरं त्रायते सुतः । ।।
तस्मात् पुत्र इति प्रोक्तः स्वयमेव स्वयम्भुवा ॥39॥
-‘पुत्र ‘पुत्’ नामक नरकसे पिताका त्राण करता है, इसलिये साक्षात् ब्रह्माजी ने उसे ‘पुत्र’ कहा है ॥
-Since a son save his father from hell named as “Put”(पुत्), therefore Lord Brahma referred him as “Putr” (पुत्र)