ὄμνυθι

From LSJ

βορβόρῳ δ' ὕδωρ λαμπρὸν μιαίνων οὔποθ' εὑρήσεις ποτόν → once limpid waters are stained with mud, you'll never find a drink

Source

French (Bailly abrégé)

2ᵉ sg. prés. impér. de ὄμνυμι.