1940s: abbreviation of pregnant + -ers.

• rugby from mid 19th century:

  The game of rugby is named after Rugby School in Warwickshire, England, the public school where it was first played. According to tradition, in a school football match in 1823 a boy named William Webb Ellis first took the ball in his arms and ran with it, so originating the game. The informal name rugger was invented at Oxford University in, it seems, 1893. At the time there was a student craze for adding –er to the end of words, which gave us words such as soccer, brekkers (for ‘breakfast’), and preggers (for ‘pregnant’), as well as some that lasted only for a year or so, like Pragger-Wagger for the Prince of Wales, and even wagger-pagger-bagger for ‘wastepaper basket’. Ironically, the craze started at Rugby School, home of rugby.