From Proto-Slavic *ne + *sǫtь, first-person plural present tense form of *byti. This formation was kept in Old Church Slavonic as не сѫтъ (ne sǫtŭ) (see бꙑти (byti)). However, in other South Slavic vernaculars, this form irregularly later created něsu (attested in 1334), probably influenced by něsam (see nisam). Because of the yat reflex, něsu became rendered as нису in Ikavian speeches.
ни́су (Latin spelling nísu)