Abarim Publications' online Biblical Greek Dictionary
The ubiquitous preposition and prefix υπο (hupo) generally means under, beneath or through. It creates a (downward) motion either from something, towards something or through (because of / under cover of) something. As such it's often translatable as 'by'.
Our particle mostly generates a genitive form of the word it points toward, but also comes with an accusative. As prefix our word occurs in a colossal array of composite words.