Rashi on Deuteronomy 32:33/רש"י על דברים ל״ב:ל״ג

1חמת תנינים יינם THEIR WINE IS THE POISON OF DRAGONS — Understand this as the Targum does: הא כמרת תניניא כס פורענותהון which means: Behold, like the gall of serpents is the cup of the drink of their punishment.

2וראש פתנים AND AS THE VENOM OF VIPERS is their cup — venom, which proves cruel (אכזר) to him who is bitten. The sense is: a cruel enemy will come and exact punishment from them.