Source : Webster's Revised Unabridged Dictionary (1913)

Leave \Leave\, n. [OE. leve, leave, AS. le['a]f; akin to le['o]f
   pleasing, dear, E. lief, D. oorlof leave, G. arlaub, and
   erlauben to permit, Icel. leyfi. ? See {Lief}.]
   1. Liberty granted by which restraint or illegality is
      removed; permission; allowance; license.

            David earnestly asked leave of me.    --1 Sam. xx.
                                                  6.

            No friend has leave to bear away the dead. --Dryden.

   2. The act of leaving or departing; a formal parting; a
      leaving; farewell; adieu; -- used chiefly in the phrase,
      to take leave, i. e., literally, to take permission to go.

            A double blessing is a'double grace; Occasion smiles
            upon a second leave.                  --Shak.

            And Paul after this tarried there yet a good while,
            and then took his leave of the brethren. --Acts
                                                  xviii. 18.

   {French leave}. See under {French}.

   Syn: See {Liberty}.

French \French\ (fr[e^]nch), a. [AS. frencisc, LL. franciscus,
   from L. Francus a Frank: cf. OF. franceis, franchois,
   fran[,c]ois, F. fran[,c]ais. See {Frank}, a., and cf.
   {Frankish}.]
   Of or pertaining to France or its inhabitants.

   {French bean} (Bot.), the common kidney bean ({Phaseolus
      vulgaris}).

   {French berry} (Bot.), the berry of a species of buckthorn
      ({Rhamnus catharticus}), which affords a saffron, green or
      purple pigment.

   {French casement} (Arch.) See {French window}, under
      {Window}.

   {French chalk} (Min.), a variety of granular talc; -- used
      for drawing lines on cloth, etc. See under {Chalk}.

   {French cowslip} (Bot.) The {Primula Auricula}. See
      {Bear's-ear}.

   {French fake} (Naut.), a mode of coiling a rope by running it
      backward and forward in parallel bends, so that it may run
      freely.

   {French honeysuckle} (Bot.) a plant of the genus {Hedysarum}
      ({H. coronarium}); -- called also {garland honeysuckle}.
      

   {French horn}, a metallic wind instrument, consisting of a
      long tube twisted into circular folds and gradually
      expanding from the mouthpiece to the end at which the
      sound issues; -- called in France {cor de chasse}.

   {French leave}, an informal, hasty, or secret departure;
      esp., the leaving a place without paying one's debts.

   {French pie} [French (here used in sense of ``foreign'') +
      pie a magpie (in allusion to its black and white color)]
      (Zo["o]l.), the European great spotted woodpecker
      ({Dryobstes major}); -- called also {wood pie}.

   {French polish}.
   (a) A preparation for the surface of woodwork, consisting of
       gums dissolved in alcohol, either shellac alone, or
       shellac with other gums added.
   (b) The glossy surface produced by the application of the
       above.

   {French purple}, a dyestuff obtained from lichens and used
      for coloring woolen and silken fabrics, without the aid of
      mordants. --Ure.

   {French red} rouge.

   {French rice}, amelcorn.

   {French roof} (Arch.), a modified form of mansard roof having
      a nearly flat deck for the upper slope.

   {French tub}, a dyer's mixture of protochloride of tin and
      logwood; -- called also {plum tub}. --Ure.

   {French window}. See under {Window}.

Source : WordNet®

French leave
     n : an abrupt and unannounced departure (without saying
         farewell)