A purchase or aquisistion that is kept in the trunk of one's car, awaiting a more oportune moment to reveal to friends, family, especially ones spouse.
Yeh, I bought the crazy expensive shoes, but they're trunkers until that party on Friday night.
In the UK refers to a large wagon like a 40 tonnes artic which shuttles up a motorway dropping bulk parcel deliveries from onemajor depot off to smaller depots at industrial areas close to the motorway. Can also refer to the wagon driver.