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ни с того ни с сего stresses
gen.	unprompted (jusska); all of sudden; without any visible reason; out of a clear sky; for no apparent reason (Anglophile); on a whim (типа заскока в голов, шарики за ролики закатились mazurov); suddenly (Pickman); out of nowhere ("Do you like Thai food?" he suddenly asked her out of nowhere. Tundra_cold); for no reason at all; without rhyme or reason; unaccountably (Asterite); without warning (VLZ_58); on the spur of the moment (EV_Ageeva); spontaneously (Alex_Odeychuk); when you least expect (Zippity); point blank; out of the blue; out of thin air; right out of the blue
Игорь Миг	off the cuff; inexplicably
amer.	all at once (All at once I felt dizzy and had to lie down. Val_Ships)
idiom.	out of left side (Eranwen); just like that (suddenly and, sometimes, unexpectedly: The alarm went off, just like that. • We were jogging slowly in the park when, just like that, she collapsed. Andrey Truhachev)
inf.	out of a clear blue sky (Ася Кудрявцева); surprisingly (Damirules); all the sudden (baletnica); out of sudden (неправильное использование, должно быть all of a sudden Damirules); unexpectedly (см. по контексту Damirules); for no particular reason (русское сочетание без запятой, см. gramota.ru Abysslooker)
Makarov.	apropos of nothing
math.	all of a sudden
proverb, disappr.	all of a sudden; for no good reason
slang	out of a clear sky
ни с того, ни с сего
gen.	plainly

ни с того ни с сего: 12 phrases in 5 subjects

American usage, not spelling	1
General	5
Idiomatic	1
Makarov	3
Slang	2