From Atharva Veda
Upanishad - Upa - Near Ni- Nishchayena(sure)
Sad
- weakening - sense of attachment & bondage,
attaining - realize you are that
destroying - avasadhana , destroying ignorance. Sense of false identification with body mind complex
https://www.wisdomlib.org/hinduism/book/mundaka-upanishad-shankara-bhashya
Mundanam - shaving the head
Mundaka - means barbers blade to shave the head or shaved head or here it might means cleaning your head of all impurities, emptying your head
Summary: The Mundaka Upanishad is a collection of philosophical poems used to teach meditation and spiritual knowledge regarding the true nature of Brahma and the Self (Atman). It is composed of the three main parts (mundakas):
1) The first of three parts expounds the science of higher and lower knowledge.
2) The second part describes the true nature of the Self (Atman) and Brahman.
3) The third part further builds on the previous two parts and attempts to describe the state of knowing Brahman, which is one of bliss and fearlessness.
This commentary by Shankara focuses on ‘Advaita Vedanta’, or non-dualism: one of the classical orthodox philosophies of Hinduism.
ओं । ब्रह्मा देवानां प्रथमः संबभूव विश्वस्यकर्ता भुवनस्य गोप्ता ।
स ब्रह्मविद्यां सर्वविद्याप्रतिष्ठामथर्वाय ज्येष्ठपुत्राय प्राह ॥ १ ॥
oṃ | brahmā devānāṃ prathamaḥ saṃbabhūva viśvasyakartā bhuvanasya goptā |
sa brahmavidyāṃ sarvavidyāpratiṣṭhāmatharvāya jyeṣṭhaputrāya prāha || 1 ||
1. Brahma was the first among the Devas, the creator of the universe, the protector of the world. He taught the knowledge of Brahman, on which all knowledge rests, to his eldest son Atharva.