UNCOUPLE is a 8 letter word that starts with U.
\Un*cou"ple\, v. t. [1st pref. un- + couple.]
To loose, as dogs, from their couples; also, to set loose; to
disconnect; to disjoin; as, to uncouple railroad cars.
\Un*cou"ple\, v. i.
To roam at liberty. [Obs.] --Chaucer.