Pinup as we know it today is radically different to its humble origins which can be traced back to the beginning of the 20th century, a time when sexuality was both scrutinized and suppressed. For instance a woman showing a bare ankle was considered risqué and the word "sex" itself was not even used publicly.
As ever people find a way round even the most stringent of rules and the desire for images of an erotic nature was overcome by the birth of portraying scantily clad ladies as an art form.
As art such images were deemed acceptable by the society of the time and many of these early drawings and illustrations that are considered the roots of the pinup genre used imagery of the burlesque striptease routines as their starting point.
Additionally magazines such as 'The Police Gazette' illustrated stories of murder and mayhem by showing the leading ladies in various states of undress, which was considered to be completely legitimate by the general populous as they were considered newsworthy.