This is the 8865th most frequent Polish word.
sandrę
The word "sandrę" does not have a direct definition in Polish; it appears to be a form or misspelling of a name ("Sandra").
In this sentence, 'sandrę' is the accusative form of the proper noun 'Sandra', referring to the name of a person whom the speaker saw by the lake.
Widziałem sandrę nad jeziorem dzisiaj.
I saw Sandra by the lake today.
Here, 'sandrę' refers to the geographical term 'sandr', which are sandy outwash plains formed by glacial activity, used in the accusative form.
Geografowie studiowali sandrę na pomorzu.
Geographers studied the sandr in Pomerania.
In this usage, 'sandrę' might signify a type of plant or perhaps decorative sand, showing a specialized or figurative application, also in accusative.