What is the definition of dharma as per Karna? I read somewhere that actions that weakens the soul is not dharma. Can anyone cite some reference on the same.
What is the definition of dharma as per Karṇa?
From this conversation between Kṛṣṇa and Karṇa from Udyoga-parva of Mahābhārata:
One should not let go of one's adopted parents if the real parents suddenly pop up one day.
Kunti, however, abandoned me without thinking of my welfare. The Suta, Adhiratha, as soon as he beheld me, took me to his home, and from her affection for me, Radha's breasts were filled with milk that very day, and she, O Madhava, cleansed my urine and evacuations. How can one like us, conversant with duties and ever engaged in listening to scriptures deprive her [Radha] of her Pinda?
So also Adhiratha of the Suta class regardeth me as a son, and I too, from affection, always regard him as (my) father. O Madhava, that Adhiratha, O Janardana, from paternal affection caused all the rites of infancy to be performed on my person, according to the rules prescribed in the scriptures. It is that Adhiratha, again, who caused the name Vasusena to be bestowed upon me by the Brahmanas. When also I attained to youth, I married wives according to his selections. Through them have been born my sons and grandsons, O Janardana. My heart also, O Krishna, and all the bonds of affection and love, are fixed on them. From joy or fear. O Govinda. I cannot venture to destroy those bonds even for the sake of the whole earth or heaps of gold.
Never betray your friend to win your blood relatives.
In consequence also of my connection with Duryodhana of Dhritarashtra's race, I have, O Krishna, enjoyed sovereignty for thirteen years, without a thorn on my side. I have performed many sacrifices, always however in connection with persons of the Suta tribe. All my family rites and marriage rites have been performed with the Sutas. Obtaining me, O Krishna, Duryodhana hath, O thou of Vrishni's race, made this preparations for an armed encounter and provoked hostilities with the sons of Pandu. And it is for this, O Achyuta, that in the battle (that will ensue), I, O Krishna, have been chosen as the great antagonist of Arjuna to advance against him in a single combat. For the sake of death, or the ties of blood, or fear, or temptation, I cannot venture, O Janardana, to behave falsely towards the intelligent son of Dhritarashtra.
I read somewhere that actions that weakens the soul is not dharma
Not sure where Karṇa says that, but if I find a reference I'll update my answer.