сутнасць

• су́тнасьць (sútnasʹcʹ) — Taraškievica orthography
• sutnaść — Łacinka (Belarusian Latin alphabet)

Uncertain. Possibly derived from one of several now-obsolete or even unattested stems:

• суць f (sucʹ, “essence; archaic third-person plural form of быць (bycʹ)”) +‎ -насць (-nascʹ), which see Russian суть (sutʹ) and су́щность (súščnostʹ);
• Or су́тні (sútni, “essential”) +‎ -асць (-ascʹ), which see Ukrainian су́тність (sútnistʹ).

Etymologically and historically, one would expect the latter, i.e. Ukrainian су́тність (sútnistʹ) to be closer to the Belarusian term, however neither *суць (*sucʹ) nor *су́тні (*sútni) are recorded in major dictionaries, with especially the latter almost exclusively found in Ukrainian sources. On the other hand, *суць (*sucʹ) may be used occasionally by some Belarusian speakers as a reconstructed cognate to Russian суть (sutʹ), however this use is not formally documented in any dictionaries.

Likewise, Russian сущий (suščij) does not have a cognate in modern Belarusian, making the formation of a direct cognate form to су́щность (súščnostʹ) theoretically impossible.

Alternatively, simply borrowed from Ukrainian су́тність (sútnistʹ).

• IPA^(key):
• Rhymes: -utnasʲt͡sʲ

су́тнасць • (sútnascʹ) f inan (genitive су́тнасці, uncountable)

1. essence

   су́тнасць жыцця́ ― sútnascʹ žyccjá ― the essence of life

   гавары́ць па су́тнасці спра́вы ― havarýcʹ pa sútnasci správy ― to talk about the essence of the matter

   па су́тнасці ― pa sútnasci ― in essence; essentially

2. (philosophy) essence, nature (the true nature of anything, not accidental or illusory)
3. entity, creature, being

• (all senses): існасць (isnascʹ)

• “сутнасць” in Belarusian–Russian dictionaries and Belarusian dictionaries at slounik.org
• “сутнасць”, in Skarnik's Belarusian dictionary (in Belarusian), based on Kandrat Krapiva's Explanatory Dictionary of the Belarusian Language (1977-1984)