From Latin precari (to pray). Ultimately from the Indo-European root prek- (to ask) that is also the source of words such as pray, precarious, deprecate, and postulate.

At first I thought precari would also have led to 'preach', but I found 'preach' comes from late O.E. predician.
It's funny that the root prek- is only two letters short for our word for preaching/sermon : n.'preek' or verb. preken.
What I remember from our youth is that preken, the preek was never ask but always tell Donner und Blitze:'thou shalt or shalt not'.

precatory and predician, almost alike visually, have then opposites meanings?