Genealogists face a constant influx of records from a different era with outdated language and terminology. When faced with evidence that your ancestor’s occupation dealt with dealbation, where would you turn next? Or if a diary states that your ancestress was fond of diet-drink, was she perpetually trying to lose weight?
The best place to find definitions of such terms is to locate a dictionary from the time and place where your ancestor lived, and read the definition. Remember that word definitions change over time, or by location (just ask a New Englander for Sweet Tea) so it is important to keep as close as possible to the time and place. The Internet Archive is a great place to find these dictionaries online. Libraries and archives will often have physical books as well. NEHGS has numerous dictionaries dating back to the eighteenth and nineteenth centuries.
For the record, dealbation concerns whitening, bleaching, or blanching. And diet-drink is a drink prescribed and prepared for medical purposes.