καπάκι – lid, just the humble cover for a pot, jar or bottle. Another borrowing, from the Turkish ‘kapak’.
It also has other meanings. For example, the καπάκι is the soundboard or resonator of a stringed instrument.
www.slang.gr also claims καπάκι can also mean a motorcycle accident, specifically the result of a doing a σούζα (wheely). “Χτες έκανα 1 χλμ σούζα στην Εθνική οδό, αλλά τελικά εφαγα καπάκι και χτύπησα!” = “Yesterday, I did a one-kilometre wheely on the main road but I came off and hurt myself.”
στο καπάκι = immediately [after]. Babiniotis gives this example: “Με το που θα τελειώσω τις σπουδές, θα πάω στο καπάκι στρατό.” = As soon as I finish Uni, I will go straight into the army.”
Φέρνω κάποιον καπάκι = to bring someone round to your way of thinking. It also seems to mean to wrap someone round your little finger. And while we are talking negatively, κύλησε ο τέντζερης και βρήκε το καπάκι literally means the pan [over]flowed and found its lid. It means something akin to “birds of a feather flock together”.
Historically, especially in the context of the period of Turkish rule (τουρκοκρατία), καπάκια were the temporary, secret deals that Greek chieftains engaged in with the Turks. During the War of Independence, Androutsos seems to have engaged in several καπάκια with the Turks to the point that his own side could no longer trust him. Here “under the counter deals” might be a reasonable translation. For a more detailed reading, see https://el.wiktionary.org/wiki/καπάκι
Finally, I was a little surprised that this important historical meaning is not covered in the Babiniotis dictionary.