[IKINARI]

It means 'Suddenly, all of sudden' in Japanese. You can also say 'KYUU-NI', and 'TOTSU-ZEN'. Those are almost same meaning, but you need to use properly. It depends on the sentence.

IKINARI = Something happens without warning

KYUU-NI = Something happens unexpectedly

TOTSU-ZEN = Almost same as both IKINARI and KYUU-NI