To fetch a thing before it is asked for. Preloading spends effort on a guess about what will be wanted.
From pre- (Latin prae, before) and load (Old English hlæstan, to put a burden aboard); to fetch a thing before it is asked for, spending effort now on a guess about what will be wanted.
To fetch a thing before it has been asked for, spending effort now on a guess about what will be wanted later.