verb

• (transitive, nautical) To shift a fore-and-aft sail from one side of a sailing vessel to the other, while sailing before the wind.
• (intransitive, nautical) Of a fore-and-aft sail or its boom: to shift, often forcefully and suddenly, from one side of a sailing vessel to the other.
  "[…] I had my Man Friday to teach as to what belong'd to the Navigation of my Boat; for though he knew very well how to paddle a Canoe, he knew nothing what belong'd to a Sail and a Rudder, and was the moſt amaz'd when he ſaw me work the Boat too and again in the Sea by the Rudder, and how the Sail gyb'd, and fill'd this Way or that Way, as the Courſe we ſail'd chang'd: […]"
• (intransitive, nautical) Generally of a small sailing vessel: to change tack with the wind crossing behind the vessel.
• (by extension, obsolete) Often as gybe at: to balk, hesitate, or vacillate when faced with a course of action, plan, or proposal.

noun

• (nautical) The act of gybing.
• (nautical) The act of gybing.
  "The key to a good gybe is to bring the spinnaker round to the old weather side before you begin, and then to steer to keep some wind in the kite."
• (by extension) A sudden change in approach or direction; vacillation.