Αἰγιαλῷ λαλεῖς.
You're talking to the seashore.
This is a proverbial fool's errand: you're talking to the waves pounding against the seashore, i.e. to someone who cannot hear you. You can see the Greek root λαλ- in the -lalia words like glossolalia and echolalia.
Ὄφιν θάλπεις.
You're warming a snake.
In English we warn people about the threat posed by a snake or viper in one's bosom, i.e. a dangerous person to whom you are close, but this Greek proverb alludes specifically to the Aesop's fable about the foolish man who found a snake half-frozen in the snow; feeling sorry for the snake, he warmed it in his cloak (or by his fireside), but when the snake revived, it bit him!
Ἄνθρωπος μέτρον.
Man (is) the measure.
The words are a fragment of the pre-Socratic philosopher Protagoras, who argued for philosophical relativism: what a person subjectively perceives is the measure of their reality. It is sometimes cited as Ἄνθρωπος μέτρον ἁπάντων, "Man (is) the measure of all things." You can see the Greek μέτρον in English metronome.
Ἐς πόδας ἐκ κεφαλῆς.
To the feet from the head.
Or, as we might say in English: "from head to foot" or "from head to toe" — although the Greek makes more sense than the English, referring to feet in the plural. The phrase is found already in Homer's Iliad, referring to Patroclus' body being prepared for his funeral. From the Greek κεφαλ- we get all the -cephal- words in English like encephalitis and cephalopod.
Ὃ γέγραφα γέγραφα.
That which I have written, I have written.
The words are spoken by Pontius Pilate in the Gospel of John. The story goes that Pilate wrote a sign to hang on the cross of Jesus: Ἰησοῦς ὁ Ναζωραῖος ὁ Βασιλεὺς τῶν ἰουδαίων, Jesus of Nazareth King of the Jews. The Jewish priests objected, but Pilate would not change what he had written. This famous incident has its own article at Wikipedia: Quod scripsi scripsi. You can also find out more at this article: Jesus, King of the Jews.
And to finish up, here's a random proverb and a random LOLCat too: