Either of two letters, little yus (Ѧ) and big yus (Ѫ), representing nasal vowel sounds in the Cyrillic alphabet. The only major Slavic language retaining these sounds is Polish, which is written in the Latin alphabet.
Wiktionary
Origin of Yus
Russianюс (jus), from Old Church Slavonic ѫсъ (ǫsъ, “big yus”)