In plain words
Tapas performed from deluded stubbornness by tormenting oneself, or for the purpose of destroying another, is declared tāmasa.
Word for word
Tapas which is done with a deluded conviction, with torment of oneself, or for the ruin of another: that is declared tāmasa.
मूढग्राहेणाऽऽत्मनो यत् पीडया क्रियते तपःपरस्योत्सादनार्थं वा तत् तामसमुदाहृतम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
मूढग्राहेण अविवेकनिश्चयेन आत्मनः पीडया यत् क्रियते तपः परस्य उत्सादनार्थं विनाशार्थं वा, तत् तामसं तपः उदाहृतम् ॥ १९ ॥
In plain words
Austerity taken up out of a deluded fixation — a resolve hardened with no discrimination behind it — carried out by torturing your own body, or aimed at ruining, at destroying, someone else: that austerity is called tāmasic.
Close to the Sanskrit
That austerity which is performed with a deluded grasping — that is, with a determination devoid of discrimination — through torment of one's own self, or for the purpose of uprooting, of destroying, another: that austerity is declared to be tāmasic.