Verb
(aviation, space travel) To allow (a reefed parachute) to fully open.
Once the SRBs decelerate following initial main-parachute deployment, the parachutes disreef in stages to gradually slow the boosters to a safe splashdown speed.
(nautical) To let out (a reefed sail) to increase the area it exposes to the wind.
Source: en.wiktionary.org