A phrase that is believed to have first been used in the town of Blackwood, South Wales by boy racers cruising up and down the high street to get the attention of good looking female members of the population.
"Get Your Rat Out" literally means expose your Vagina or Vulva. In the late 1990's it became apparent that "Excuse me Madame, but would you mind showing me your vaginal opening, only I'm rather fond of you and all that" was no longer acceptable, hence a much shorter phrase was required. Recently, since 2004 "Get Your Rat Out" has been shortened to just "Rat Out". Academics have theorised that within the next 5 years this will be shortened to just "Rat", as motor vehicles become faster thus making it more difficult for one to shout “Rat Out” whilst passing some rather delicious ladies.