Θάλαττα! θάλαττα!
The sea! The sea!
This was the cry of Cyrus's army of Greek mercenaries who had been stranded in Persia and finally reached the Black Sea after a two-year, 3000-mile journey; the story is told in Xenophon's Anabasis. It features the Attic pronunciation: θάλαττα instead of θάλασσα. The phrase has its own Wikipedia article: Thalatta! Thalatta!
Ἄμμον μετρεῖς.
You're measuring sand.
This is a proverbial fool's errand; the idea is that you are counting grains of sand... which means you will never stop counting. From the root in Greek μετρ- we get all the meter and -metry words in English, like kilometer and geometry.
Ἐξ ἑνὸς πηλοῦ.
From one clay.
This saying is used to refer to two people who are similar because they were made "from one clay," i.e. like pots made from the same clay. In one Greek account of the creation, Prometheus shaped human beings from clay (Wikipedia). The same idea also appears in the Biblical Book of Wisdom — ἐκ τοῦ αὐτοῦ πηλοῦ, "from the same clay" —as God also was supposed to have made human beings from the clay of the earth.
Εἰδὼς σίγα.
Having learned something, keep it quiet.
These words are attributed to Solon, one of the Seven Sages of Greece; find out more at Wikipedia. The word εἰδὼς is a perfect participle from οἶδα, a verb that only has a perfect system; a good way to see how that works is at the Wiktionary.
Βοιωτία ὗς.
A Boeotian pig.
This saying brings together two proverbially ignorant creatures: someone from Boeotia, and a pig. Combine the two, and you've got someone who is totally ignorant. You've already seen a proverb mocking the people of Boeotia in a previous post: Βοιώτιος νοῦς. You've also seen a saying mocking the ignorance of the foolish pig who dares to rival the goddess of wisdom in this post: Ὗς πότ᾽ Ἀθηναίαν ἔριν ἤρισεν.
And to finish up, here's a random proverb and a random LOLCat too: