The Collaborative International Dictionary of English v.0.48:

Rover \Rov"er\, n. [D. roover a robber. See Rove, v. i.]
   1. One who practices robbery on the seas; a pirate.
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            Yet Pompey the Great deserveth honor more justly for
            scouring the seas, and taking from the rovers 846
            sail of ships.                        --Holland.
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   2. One who wanders about by sea or land; a wanderer; a
      rambler.
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   3. Hence, a fickle, inconstant person.
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   4. (Croquet) A ball which has passed through all the hoops
      and would go out if it hit the stake but is continued in
      play; also, the player of such a ball.
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   5. (Archery)
      (a) Casual marks at uncertain distances. --Encyc. Brit.
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      (b) A sort of arrow. [Obs.]
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                All sorts, flights, rovers, and butt shafts.
                                                  --B. Jonson.
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   At rovers, at casual marks; hence, at random; as, shooting
      at rovers. See def. 5
      (a) above. --Addison.
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                Bound down on every side with many bands because
                it shall not run at rovers.       --Robynson
                                                  (More's
                                                  Utopia).
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WordNet (r) 3.0 (2006):

rover
    n 1: someone who leads a wandering unsettled life [syn:
         wanderer, roamer, rover, bird of passage]
    2: an adult member of the Boy Scouts movement [syn: rover,
       scouter]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "rover":
   Ahasuerus, Ancient Mariner, Argonaut, Blackbeard, Captain Hook,
   Captain Kidd, Flying Dutchman, Goliard, Henry Morgan, Jean Lafitte,
   Odysseus, Oisin, Ossian, Ulysses, air pirate, airplane hijacker,
   bird of passage, buccaneer, bum, corsair, drifter, floater,
   freebooter, gad, gadabout, go-about, gypsy, hobo, itinerant, mover,
   nomad, peregrinator, peregrine, peripatetic, picaroon, pirate,
   privateer, rambler, roamer, rolling stone, runabout, sea dog,
   sea king, sea robber, sea rover, sea wolf, skyjacker, sojourner,
   straggler, stroller, strolling player, tourist, tramp, traveller,
   troubadour, vagabond, vagrant, viking, visitant, wanderer,
   wandering minstrel, wandering scholar, wayfarer