нельзя

Russian edit

From не (ne) + льзя (lʹzja), from Old East Slavic нельзѣ (nelĭzě), нельзѧ (nelĭzę), нельза (nelĭza), from Proto-Slavic *ne / *ně + *lьga (“possibility”) (possibly influenced by its dative *lьdzě), likely related to *lьgъkъ (whence лёгкий (ljóxkij, “light, easy”)).

Compare Ukrainian не́льга (nélʹha), нільга́ (nilʹhá), нельзя́ (nelʹzjá), Belarusian няльга́ (njalʹhá), нільга́ (nilʹhá), не́льга (njélʹha), Old Church Slavonic льзѣ (lĭzě, “possible, possibly”), Slovak nelza, nelzä, neľzä, Old Polish nielza, also modern Slovene lahko (“may, be allowed”)

нельзя́ • (nelʹzjá) (impersonal, invariable)

it is impossible (perfective aspect), one cannot, you can't
нельзя́ ему́ + perfective infinitive ― nelʹzjá jemú + perfective infinitive ― it is impossible for him to (perfective infinitive)
it is prohibited (imperfective aspect), it is forbidden, it is not allowed, one must not
здесь нельзя́ кури́ть ― zdesʹ nelʹzjá kurítʹ ― it is not allowed to smoke here
one cannot
нельзя́ не заме́тить, что он влюби́лся в неё ― nelʹzjá ne zamétitʹ, što on vljubílsja v nejó ― One cannot fail to notice that he has fallen in love with her
not an option
Проиграть нельзя́. ― Proigratʹ nelʹzjá. ― Losing is not an option.

Нельзя́ is the negative of мо́жно (móžno).