īshwī     no! not at all!
no    oya
(adj.) nta, ata
(emphatic) oyāye, ishwi, eka, ekāye, wapi
(to say ~) guhakana
(euphemism for a polite No) nzokubarira (lit. I will let you know)
En-En dictionary 
not    nta, ata
(is not, are not) si
(not at all) na mba! īshwī!
(not a bit) n'intete
En-En dictionary 
 ishwi found in: Kirundi II (Grammar)
chapter 76   Affirmative and Negative Words