In plain words
A yajña without proper rule, without distribution of food, without mantra, without offering to the officiants, and without faith is called tāmasa.
Word for word
Without injunction, with no food given, without mantra, without dakṣiṇā, devoid of śraddhā: [such] a yajña they call tāmasa.
विधिहीनमसृष्टान्नं मन्त्रहीनमदक्षिणम्श्रद्धाविरहितं यज्ञं तामसं परिचक्षते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
विधिहीनं यथाचोदितविपरीतम् , असृष्टान्नं ब्राह्मणेभ्यो न सृष्टं न दत्तम् अन्नं यस्मिन् यज्ञे सः असृष्टान्नः तम् असृष्टान्नम् , मन्त्रहीनं मन्त्रतः स्वरतो वर्णतो वा वियुक्तं मन्त्रहीनम् , अदक्षिणम् उक्तदक्षिणारहितम् , श्रद्धाविरहितं यज्ञं तामसं परिचक्षते तमोनिर्वृत्तं कथयन्ति ॥ १३ ॥
In plain words
'Vidhihina' means exactly the opposite of what is prescribed. 'Asrstanna' -- a sacrifice where food is never actually given to the brahmanas, never offered -- that is what makes it 'food not given.' 'Mantrahina' means the mantras are wrong, whether in tone or in pronunciation. 'Adaksina' means the required fee is simply missing. And a sacrifice with no faith behind it at all -- that is what people call tamasic, born out of tamas.
Close to the Sanskrit
'Vidhihina' [means] the opposite of what has been enjoined; 'asrstanna' -- [that sacrifice] in which food has not been given, not offered, to brahmanas -- that is 'asrstanna,' 'food not given'; 'mantrahina' [means] devoid of mantra, whether by tone or by pronunciation; 'adaksina' [means] devoid of the stated sacrificial fee; a sacrifice devoid of faith -- they call it tamasic, they say it is produced from tamas.