Rashi on Deuteronomy 29:17/רש"י על דברים כ״ט:י״ז

1פן יש בכם means PERHAPS (שמא = פן) THERE IS AMONG YOU,

2אשר לבבו פנה היום [A MAN …] WHOSE HEART TURNETH AWAY THIS DAY from accepting the covenant upon himself,

3שרש פרה ראש ולענה [PERHAPS THERE IS AMONG YOU] A ROOT THAT IS FRUITFUL IN POISONOUS HERB AND WORMWOOD — i.e. a root that brings forth herbs bitter as wormwood-plants, which are very bitter. The meaning is: Lest there be a man or woman or family or tribe that fruitfully produces and increases wickedness in your midst.