Ooh, re: men/boys being called "boy" rather than their name or a shortened version of their name....I actually have a relative who was called "boy" 90% of the time by his family!!! In this instance (really weird, but you can see why), it was actually because they were so chuffed to have a boy (yes, he was youngest and a bit spoiled) - his poor sis, who did get called by her real name all the time. His sister was the only one who used his real name significantly more than anyone else; even relatives were encouraged to call him "boy"!! When he went to uni he decided to give himself an extra first name that he insisted his friends and his relatives called him by from then on, even though his documents had his real name. The family don't call him boy any more.
Re: the Cissy story, that seems to be a common derivation of the name...I know of someone who was called that because her brother couldn't say "sister", and soon everyone used it as a nickname too. She liked it so much she used it thereafter, and left her conventional (or so she thought) first and middle names just for official documents.