Jackknife meaning : A compact folding knife. The front-dive pike, in which the body folds and unfolds. A semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife. To fold in the middle, as a jackknife does. To cause a semi-trailer truck to fold like a jackknife in a traffic accident. A resampling method that applies estimators to all subsamples that each omit a single different group (possibly of a single datapoint) of the original sample to provide a sample distribution of the estimate.