to insinuate, unfairly accuse ("jemandem etwas unterstellen")
to assume, presume (Only in certain phrasings, but you'll find it in daily life.)
"unterstellen" is both, separable and non-separable. Today, we'll learn which will help you against rain and which is generally not nice. Also: runterstellen.
unterstellen, sich unterstellen, die Unterstellung