योगवासिष्ठः yogavāsiṣṭhaḥ - book-6, chapter-126, verse-18

इत्थंभूतमतिः शास्त्रगुरुसज्जनसेवनात् ।
सरहस्यमशेषेण यथावदधिगच्छति ॥ १८ ॥

itthaṃbhūtamatiḥ śāstragurusajjanasevanāt ,
sarahasyamaśeṣeṇa yathāvadadhigacchati 18

18. itthaṃbhūtamatiḥ śāstragurusajjanasevanāt
sarahasyam aśeṣeṇa yathāvat adhigacchati

18. itthaṃbhūtamatiḥ śāstragurusajjanasevanāt
sarahasyam aśeṣeṇa yathāvat adhigacchati

18. A person whose mind has become thus (prepared) fully and accurately comprehends the truth, including its deeper mysteries, by serving the scriptures, spiritual teachers (guru), and virtuous individuals.

Words meanings summery:

• इत्थंभूतमतिः (itthaṁbhūtamatiḥ) - one whose intellect/mind is thus, having such a mind

• शास्त्रगुरुसज्जनसेवनात् (śāstragurusajjanasevanāt) - from the service of scriptures, spiritual teachers, and virtuous individuals

• सरहस्यम् (sarahasyam) - along with its secret/mystery, esoteric

• अशेषेण (aśeṣeṇa) - completely, entirely, without remainder

• यथावत् (yathāvat) - properly, accurately, as it is, truly

• अधिगच्छति (adhigacchati) - comprehends, understands, obtains, reaches

Words meanings and morphology

इत्थंभूतमतिः (itthaṁbhūtamatiḥ) - one whose intellect/mind is thus, having such a mind

(noun)

Nominative, masculine, singular of itthaṃbhūtamati

itthaṁbhūtamati - one whose intellect/mind is thus, having such a mind

Compound type : bahuvrīhi (itthaṃbhūta+mati)

• itthaṃbhūta – having become thus, being of such a nature
  adjective
  Past Passive Participle
  From verb root bhū with adverb itthaṃ (thus)
  Root: bhū (class 1)
• mati – mind, intellect, thought, intention
  noun (feminine)

शास्त्रगुरुसज्जनसेवनात् (śāstragurusajjanasevanāt) - from the service of scriptures, spiritual teachers, and virtuous individuals

(noun)

Ablative, neuter, singular of śāstragurusajjanasevana

śāstragurusajjanasevana - service to scriptures, spiritual teachers, and virtuous individuals

Compound type : dvandva-tatpuruṣa (śāstra+guru+sajjana+sevana)

• śāstra – scripture, sacred text, treatise, rule
  noun (neuter)
• guru – spiritual teacher, preceptor, heavy, important
  noun (masculine)
• sajjana – good person, virtuous individual
  noun (masculine)
• sevana – service, serving, attendance
  noun (neuter)
  action noun
  Derived from verb root sev-
  Root: sev (class 1)

सरहस्यम् (sarahasyam) - along with its secret/mystery, esoteric

(adjective)

Accusative, neuter, singular of sarahasya

sarahasya - with its secret, mystic

Compound type : bahuvrīhi (sa+rahasya)

• sa – with, accompanied by
  indeclinable
• rahasya – secret, mystery, privacy
  noun (neuter)

Note: Used adverbially here or as accusative object complementing the understanding of 'truth'.

अशेषेण (aśeṣeṇa) - completely, entirely, without remainder

(adjective)

Instrumental, masculine, singular of aśeṣa

aśeṣa - entire, complete, without remainder

Compound type : nañ-tatpuruṣa (a+śeṣa)

• a – not, un-
  indeclinable
• śeṣa – remainder, residue, rest
  noun (masculine)

Note: Used adverbially (by all, without remainder).

यथावत् (yathāvat) - properly, accurately, as it is, truly

(indeclinable)

अधिगच्छति (adhigacchati) - comprehends, understands, obtains, reaches

(verb)

3rd person , singular, active, present indicative (laṭ) of adhigam

Prefix: adhi

Root: gam (class 1)