ABT and UNK are abbreviations commonly used in family trees, for "about" (when you're not sure of a date) and "unknown" (when you don't know a name or location).

ABT UNK (http://abt-unk.blogspot.com) is the genealogy and family history blog of Amanda Pape. It was started December 1, 2009. This book contains a selection of posts from then through May 21, 2012.