Met de mond vol tanden staan

Literally: To stand with your mouth full of teeth

Example: “Toen de juf naar het antwoord vroeg, stond hij met de mond vol tanden.” — When the teacher asked for the answer, he was lost for words.

English equivalent: To be lost for words

Origin: A joke, as Onze Taal explains: a mouth full of teeth has no room for a tongue, and without a tongue one cannot speak. Stoett glosses it as "only teeth and no tongue." The phrase is known from the 17th century: a political pamphlet of 1612 has daer staen wy dan en kijcken met een mondt vol tanden (there we stand gaping, with a mouth full of teeth).