Da is, according to Diez, a shortened form of diva, an exclamation composed of the two imperatives dis and va: diva > dea

Da's a lot of folks what preach all day An' always pointing' out de way, Dey say dat prayin' all de time An' keepin' yo' heart all full of rhyme Will lead yo' soul to heights above Whah angels coo like a turtledove.

Nu da.... Of course these are very troublesome times, and as you say things have to move in a hurry.

Da' 's always a ba-ad sign.

Das is nicht gut.