योगवासिष्ठः yogavāsiṣṭhaḥ - book-7, chapter-166, verse-16

अन्तस्तस्यास्तु हृदये भूतधातुविवर्जिते ।
निबिडानन्तकठिना वज्रसाराऽविनाशिनी ॥ १६ ॥

antastasyāstu hṛdaye bhūtadhātuvivarjite ,
nibiḍānantakaṭhinā vajrasārā'vināśinī 16

16. antaḥ tasyāḥ tu hṛdaye bhūta-dhātu-vivarjite
nibiḍānanta-kaṭhinā vajra-sārā avināśinī

16. tu tasyāḥ hṛdaye bhūta-dhātu-vivarjite antaḥ
nibiḍānanta-kaṭhinā vajra-sārā avināśinī

16. But within its heart (hṛdaya), which is devoid of the gross elements (bhūta) and primary constituents (dhātu), it is dense, boundlessly firm, adamantine, and imperishable.

Words meanings summery:

• अन्तः (antaḥ) - inside, within, in the interior

• तस्याः (tasyāḥ) - its (referring to the feminine entity) (her, its)

• तु (tu) - but, however

• हृदये (hṛdaye) - in the heart, in the core

• भूत-धातु-विवर्जिते (bhūta-dhātu-vivarjite) - devoid of elements and constituent principles

• निबिडानन्त-कठिना (nibiḍānanta-kaṭhinā) - dense, boundless, and hard/firm

• वज्र-सारा (vajra-sārā) - adamantine, diamond-hard

• अविनाशिनी (avināśinī) - imperishable, indestructible

Words meanings and morphology

अन्तः (antaḥ) - inside, within, in the interior

(indeclinable)

तस्याः (tasyāḥ) - its (referring to the feminine entity) (her, its)

(pronoun)

Genitive, feminine, singular of tad

tad - that, she, it

तु (tu) - but, however

(indeclinable)

हृदये (hṛdaye) - in the heart, in the core

(noun)

Locative, neuter, singular of hṛdaya

hṛdaya - heart, mind, core, essence

from hṛ (to take, carry) + daya

Root: hṛ (class 1)

भूत-धातु-विवर्जिते (bhūta-dhātu-vivarjite) - devoid of elements and constituent principles

(adjective)

Locative, neuter, singular of bhūta-dhātu-vivarjita

bhūta-dhātu-vivarjita - devoid of material elements (bhūta) and primary constituents (dhātu)

vivarjita (Past Passive Participle) from vi-vṛj (to abandon, exclude)

Compound type : Tatpurusha/Bahuvrihi (bhūta+dhātu+vivarjita)

• bhūta – element (gross), being, creature, what has been
  noun/adjective (neuter)
  Past Passive Participle
  from bhū (to be, become)
  Root: bhū (class 1)
• dhātu – element, constituent, ingredient, primary substance
  noun (masculine)
  from dhā (to place, put)
  Root: dhā (class 3)
• vivarjita – excluded, shunned, abandoned, devoid of, free from
  adjective
  Past Passive Participle
  from vi-vṛj (to leave, abandon)
  Prefix: vi
  Root: vṛj (class 1)

Note: Compound adjective modifying hṛdaye.

निबिडानन्त-कठिना (nibiḍānanta-kaṭhinā) - dense, boundless, and hard/firm

(adjective)

Nominative, feminine, singular of nibiḍānanta-kaṭhina

nibiḍānanta-kaṭhina - dense, infinitely hard; extremely compact and firm

Compound type : Karmadhāraya (nibiḍa+ananta+kaṭhina)

• nibiḍa – dense, compact, firm, thick
  adjective
• ananta – endless, infinite, boundless, eternal
  adjective
  a (not) + anta (end)
  Prefix: a
  Root: am (class 1)
• kaṭhina – hard, firm, rigid, difficult
  adjective

वज्र-सारा (vajra-sārā) - adamantine, diamond-hard

(adjective)

Nominative, feminine, singular of vajra-sāra

vajra-sāra - diamond-hard, adamantine; essence of a thunderbolt

Compound type : Bahuvrihi (vajra+sāra)

• vajra – thunderbolt, diamond
  noun (neuter)
• sāra – essence, core, strength, hard part
  noun (masculine)
  from sṛ (to go, flow)
  Root: sṛ (class 1)

अविनाशिनी (avināśinī) - imperishable, indestructible

(adjective)

Nominative, feminine, singular of avināśin

avināśin - imperishable, indestructible, undecaying

from a-vināś + in (suffix for agent/possessor)

a (not) + vi-naś (to perish) + in suffix

Prefixes: a+vi

Root: naś (class 4)