In plain words
Those who practice harsh tapas not enjoined by śāstra, joined with hypocrisy and ahaṅkāra, impelled by the force of desire and attachment,
Word for word
Those people who undergo dreadful tapas not enjoined by śāstra, joined with hypocrisy and ahaṅkāra, endowed with the force of desire and attachment,
अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाःदम्भाहङ्कारसंयुक्ताः कामरागबलान्विताः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
एवं कार्यतो निर्णीताः सत्त्वादिनिष्ठाः शास्त्रविध्युत्सर्गे । तत्र कश्चिदेव सहस्रेषु देवपूजादिपरः सत्त्वनिष्ठो भवति, बाहुल्येन तु रजोनिष्ठाः तमोनिष्ठाश्चैव प्राणिनो भवन्ति । कथम् ? —
अशास्त्रविहितं न शास्त्रविहितम् अशास्त्रविहितं घोरं पीडाकरं प्राणिनाम् आत्मनश्च तपः तप्यन्ते निर्वर्तयन्ति ये जनाः ते च दम्भाहङ्कारसंयुक्ताः, दम्भश्च अहङ्कारश्च दम्भाहङ्कारौ, ताभ्यां संयुक्ताः दम्भाहङ्कारसंयुक्ताः, कामरागबलान्विताः कामश्च रागश्च कामरागौ तत्कृतं बलं कामरागबलं तेन अन्विताः कामरागबलान्विताः ॥ ५ ॥
In plain words
So the three kinds — those grounded in sattva and the rest — have been sorted out by what they actually do, in this question of setting the scriptural rule aside. And among them, only one in thousands, given over to worshipping the gods and such acts, is grounded in sattva; for the most part living beings are grounded in rajas, or in tamas. How does that show? —
'Not sanctioned by scripture' simply means: not what scripture actually enjoins. 'Terrible' means it causes real pain -- to other beings, and to the person doing it. That is the austerity in question. 'They practice' means they carry it out -- these are the people who do this, and they are 'caught up in hypocrisy and self-importance': hypocrisy and self-importance together, bound up with both of those. And they are 'driven by the force of desire and attachment': desire and attachment together, generating a force, and it is that very force that drives them.
Close to the Sanskrit
Thus, by their effects, those grounded in sattva and the rest have been determined, in the matter of the setting aside of scriptural injunction. There, only some one among thousands, devoted to the worship of the gods and the like, is grounded in sattva; but for the most part, living beings are grounded in rajas and grounded in tamas indeed. How? —
'Not enjoined by scripture' -- meaning, not enjoined by scripture -- 'terrible,' i.e. causing pain to living beings and to oneself -- 'austerity'; 'they practice austerity,' i.e. they perform, those people, and they are 'joined with hypocrisy and egotism': hypocrisy and egotism [as a pair]; joined with those two -- 'joined with hypocrisy and egotism.' 'Endowed with the force of desire and passion': desire and passion [as a pair]; the force produced by that [pair is] 'the force of desire and passion'; endowed with that -- 'endowed with the force of desire and passion.'