This is the 17442nd most frequent Polish word.
zastanawialiście
"You (plural) were wondering."
This is a question in the past tense, using 'zastanawialiście' to address a group of people informally about their opinion or consideration.
Czy zastanawialiście się nad wyborem tego filmu?
Have you considered choosing this movie?
The word 'zastanawialiście' here indicates an action that a group was performing, namely reflecting or thinking.
Zastanawialiście się, co zrobić w takiej sytuacji.
You have been thinking about what to do in such a situation.
This negative question uses 'zastanawialiście' to inquire if a group has not considered a subject.