وَكَأْسًا دِهَاقًا
Wa ka'san di haaqa
and an overflowing cup.
and a cup filled to the brim.
A cup earns the Arabic word kaʾs while there is drink in it, so the fullness is carried twice, once by the noun and again by dihāq, brimming. The line straight after strips away what a shared cup usually loosens, ruling out empty speech and lying, so the drink arrives without the ordinary consequences for the tongue.