We are not entirely sure, though it does look like it's about to 'do something', i.e. bloom in some way now.
Are the stems square and do the crushed leaves smell fragrant?
We wonder about something in the mint family, perhaps a type of perilla.
All of those plants can quickly invade and become invasive.
When it blooms, search 'perilla', or send us another photo and we'll try again.