wielbasement
Wielbasement is a term that appears to be a portmanteau, combining the Polish word "wielki" meaning "great" or "big" with the English word "basement." Literally translated, it suggests a large or significant basement. However, without further context, its exact meaning and usage are ambiguous. It is not a recognized architectural term in common parlance, nor does it appear to be a widely adopted slang term in either Polish or English.
It is possible that "wielbasement" is a specific, perhaps informal, designation used within a particular community,