This is the 1425th most frequent Russian word.
дер
"Дер" is not a standalone word in Russian; it may be a fragment or slang.
In this sentence, 'дер' is used as a poetic, colloquial abbreviation for 'дерево', vaguely representing a forested area or trees.
Он жил в избе за большим лесным дером.
He lived in a hut behind a big forest patch.
Here, 'дер' serves as the stem base for the word 'дерей' ( деревья in non-genitive plural), signifying trees in a rural setting, albeit slightly outdated or regional usage.
На школьной выставке представлены модели дерей деревни.
Models of the trees of the village are presented at the school exhibit.
In this context, 'дер' reflects an informal abbreviation of 'дерево', used in urgent or imperative speech for simplicity.