Means 'yes', 'exactly', 'right', 'correct', or 'affirmative'. The first half, 'exacta', is from the Spanish word 'exacta', meaning 'exactly'. The second half, 'malaria', is a quantification of the degree of exaction that one's exacting maintains, as well as the precision of its localized exactability.
The intensity of 'exact' is likened, thus, to the deadliness of malaria, a debilitating disease.
The precision of 'exact' is likened, thus, to either the small stature of a mosquito, or the surgical precision the insect itself uses for the extraction of blood from its victims.
"You'll never guess what happened..."
"No really, guess what happened when I waded out into the damned mosquito-infested swamp. Really think about it."
"You... got bit by a mosquito?"
"Exactamalaria! Imagine that! Next time? Why don't you go get your own damn hat!"