You've only seen it here, if I may dare to assume :) It's an affectionate reference to our esteemed member Π2, πι δύο, whose name is jokingly meshed with πιδί μ', a local pronunciation variation of παιδί μου. Π2 μου -> πι δύο μου -> Πιδίμ'. Για αποφυγή μελλοντικών αποριών, τον λέμε και πιδύε :)