сул үгThird-person possessive particle; Topic marker; one (a particular thing or person, qualified with an adjective); what (marking a subject clause)
Үгийн гарал
From Classical Mongolian ᠨᠢ (ni), as well postposed ᠢᠨᠤ (inu), ᠠᠨᠤ (anu), from postposed Middle Mongol ᠢᠨ᠌ᠤ (inu, 3sg-poss) and ᠠᠨ᠌ᠤ (anu, 3pl-poss).
-н (-n) was an oblique suffix originally after a personal pronoun’s stem, while -ь (-ʹ) is the unique suffix for possessive particles. Original determiners were merged into a single particle, so morphemes i and a which denote grammatical numbers, were omitted.
Compare also Buryat -нь (-nʹ), -ынь (-ynʹ), -иинь (-iinʹ) and Kalmyk -нь (-nʹ).
Эшлэл: Batsuren, K., Ganbold, A., Chagnaa, A., Giunchiglia, F. Building The Mongolian WordNet. Proceedings of the 10th Global WordNet Conference (GWC), p. 238, 2019.