During the cracking of the Enigma machine cillies were the nickname of predictable message-keys. These were such things as pressing three keys in a row on the Enigma keyboard (often done by an operator in a hurry). Another would be the initials of the German operator or his girlfriend. This was perhaps the basis of the name, from the often used key C.I.L. Before the English cryptanalysts would try to crack the message they would often try one of these, which would sometimes pay off.