Meaning:
Lit. “to be powdered” – as in “powdered milk” or instant cocoa. This seems to be a metaphor for a state of not being fully prepared as the above don’t become beverages until dissolved in water. You will find many expressions containing the above like: “zupa w proszku”, “mleko w proszku” and even “oranżada w proszku“.
In most cases, you use the above to point that somebody or a group of people are far from being prepared for something: a meeting with a client, an exam, not packed up for a holiday. It might also refer to enterprises.