Even if the context is missing, I'm quite sure it is like baby talking for the phrase "Não acredito" (I don't believe). Small children often pronounce the word 'acredito' in this way (quidito). Is happens because the expression 'Não acredito' is widely used as a form of surprise about something have been said. In oral language the adult pronounciation is "NUM CRIDITO". So, as children take long to learn the pronounciation of sylabes with digraphs that contains R (CR, DR, TR...), they miss the R sound and say QUI.