Today, 'Backfisch' is basically only used to describe fish fried in some kind of batter. My granny (born in 1912) on the other hand still used it to refer to teenage girls, usually rather silly ones. Lydia in Jane Austen's "Pride and Prejudice" would be a good example for that kind of Backfisch.
As you are familiar with the use of online translators, maybe these links will be helpful?
Wikipedia Backfisch_Mädchen
https://www.wortbedeutung.info/Backfisch/