The phrase "the cut of your jib" comes from real naval reconnaissance — experienced sailors could identify enemy ships by the distinctive shape of their jib sails.
A triangular sail set ahead of the foremost mast of a sailing vessel; also, the projecting arm of a crane.
Origin uncertain, possibly from Dutch gijben (to shift a sail from one side to the other) or from the earlier English jibb (to move restlessly). The nautical term may be connected to the verb jibe (to shift a sail). The crane sense extends the image of a projecting arm. Key roots
The phrase "I don't like the cut of his jib" — meaning you don't trust someone's appearance — comes from the fact that different nations rigged their jibs differently. An experienced sailor could identify a ship's nationality from the shape of its jib before any flag was visible, allowing early assessment of whether an approaching vessel was friend or foe. The crane jib takes its name from the visual similarity