The Collaborative International Dictionary of English v.0.48:

Pester \Pes"ter\ (p[e^]s"t[~e]r), v. t. [imp. & p. p.
   Pestered; p. pr. & vb. n. Pestering.] [Abbrev. fr.
   impester, fr. OF. empaistrier, empestrer, to entangle the
   feet or legs, to embarrass, F. emp[^e]trer; pref. em-, en-
   (L. in in) + LL. pastorium, pastoria, a fetter by which
   horses are prevented from wandering in the pastures, fr. L.
   pastorius belonging to a herdsman or shepherd, pastor a
   herdsman. See In, and Pasture, Pastor.]
   1. To trouble; to disturb; to annoy; to harass with petty
      vexations.
      [1913 Webster]

            We are pestered with mice and rats.   --Dr. H. More.
      [1913 Webster]

            A multitude of scribblers daily pester the world.
                                                  --Dryden.
      [1913 Webster]

   2. To crowd together in an annoying way; to overcrowd; to
      infest. [Obs.] --Milton.
      [1913 Webster]

            All rivers and pools . . . pestered full with
            fishes.                               --Holland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pester
    v 1: annoy persistently; "The children teased the boy because of
         his stammer" [syn: tease, badger, pester, bug,
         beleaguer]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "pester":
   aggravate, annoy, apply pressure, badger, bait, be at, bedevil,
   beleaguer, beset, besetment, besiege, blandish, bother,
   botheration, bristle, brown off, bug, bullyrag, burn up,
   buttonhole, cajole, carp at, chafe, chivy, coax, devil, discompose,
   distemper, disturb, dog, dun, exasperate, exasperation, exercise,
   exert pressure, fash, fret, fret at, fuss at, get, get at, get to,
   gripe, harass, harry, hassle, heckle, hector, henpeck, hound,
   importune, irk, irritant, irritate, miff, molest, nag, nag at,
   needle, nettle, nibble at, niggle, nudzh, nuisance, peck at, peeve,
   persecute, pest, pick at, pick on, pique, plague, pluck the beard,
   ply, pother, press, pressure, provoke, push, ride, rile, roil,
   ruffle, tantalize, tease, torment, try the patience,
   tweak the nose, urge, vex, wheedle, work on, worry, yap at