In places like the United Kingdom, a hip flask defence is a defense to an allegation of drunk driving that a driver had consumed alcohol between the time of a vehicular offense (such as an accident) and a breathalyser test so that a positive result does not actually indicate that they were driving while intoxicated.
Though popular (in 1991, the defence was used in 90% of Swedish appeals against drink driving convictions), the hip flask defence is not always effective. It is possible to back calculate the amount of alcohol and prove an offence nevertheless.
References
Hip flask defence Wikipedia(Text) CC BY-SA