Pliny, Natural History 22
“A poppy is boiled and consumed for insomnia. The same water is used for the face. Poppies grow best in dry conditions where it does not often rain. When the heads themselves are boiled with the leaves, the juice is called meconium and is a lot less potent than opium.”
decoquitur et bibitur contra vigilias, eademque aqua fovent ora. optimum in siccis et ubi raro pluat. cum capita ipsa et folia decocuntur, sucus meconium vocatur multum opio ignavior.
Aristotle, Historia Animalium 587a 31
“[Newborns] also discharge excrement right away, pretty soon, or at least within the same day. This material is greater than one might expect from the size of the infant and the women call it “poppy-juice” [mêkonion]. Its color is similar to blood but very dark and like pitch. Later on, it is milk-like once the baby immediately eats from the breast. Before it comes out, the newborn does not cry, even if the birth is difficult and the head sticks out while the whole body is inside.”
ἀφίησι δὲ καὶ περιττώματα τὰ μὲν εὐθὺς τὰ δὲ διὰ ταχέων, πάντα δ᾿ ἐν ἡμέρᾳ· καὶ τοῦτο τὸ περίττωμα πλέον ἢ τοῦ παιδὸς κατὰ μέγεθος· ὃ καλοῦσιν αἱ γυναῖκες μηκώνιον. χρῶμα δὲ τούτου αἱματῶδες καὶ σφόδρα μέλαν καὶ πιττῶδες, μετὰ δὲ τοῦτο ἤδη γαλακτῶδες· σπᾷ γὰρ εὐθὺς καὶ τὸν μαστόν. πρὶν δ᾿ ἐξελθεῖν οὐ φθέγγεται τὸ παιδίον, κἂν δυστοκούσης τὴν κεφαλὴν μὲν ὑπερέχῃ 35τὸ δ᾿ ὅλον σῶμα ἔχῃ ἐντός.
If you want more words for excrement in ancient Greek, we have you covered.
After oxidation, the juice of a poppy turns from white to, well, this:
Etymology of Mêkôn from Beekes (2010):