The Collaborative International Dictionary of English v.0.48:

Insistent \In*sist"ent\, a. [L. insistens, -entis, p. pr. of
   insistere.]
   [1913 Webster]
   1. Standing or resting on something; as, an insistent wall.
      --Sir H. Wotton.
      [1913 Webster]

   2. Insisting; persistent; persevering.
      [1913 Webster]

   3. (Zool.) See Incumbent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insistent
    adj 1: repetitive and persistent; "the bluejay's insistent cry"
           [syn: insistent, repetitive]
    2: demanding attention; "clamant needs"; "a crying need";
       "regarded literary questions as exigent and momentous"-
       H.L.Mencken; "insistent hunger"; "an instant need" [syn:
       clamant, crying, exigent, insistent, instant]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "insistent":
   acute, assertive, assiduous, burning, cajoling, clamant, clamorous,
   coaxing, compelling, constant, continuing, critical, crucial,
   crying, demanding, determined, diligent, dire, dogged, draining,
   dunning, emphatic, enduring, exacting, exigent, exorbitant,
   extortionate, faithful, firm, forceful, grasping, high-pressure,
   high-priority, immutable, imperative, imperious, importunate,
   inalterable, indefatigable, indomitable, industrious, inexorable,
   instant, invincible, lasting, loud, loyal, nagging, never-tiring,
   obstinate, obtrusive, patient, patient as Job, permanent,
   perseverant, persevering, persistent, persisting, pertinacious,
   pestering, pivotal, plaguing, plodding, plugging, preoccupied,
   pressing, rapt, relentless, resolute, resounding, sedulous,
   single-minded, sleepless, slogging, stable, steadfast, steady,
   stubborn, taxing, teasing, tenacious, tireless, unabating,
   uncompromising, unconquerable, undaunted, undiscouraged,
   undrooping, unfailing, unfaltering, unflagging, unflinching,
   unintermitting, uninterrupted, unnodding, unrelaxing, unrelenting,
   unremitting, unsleeping, unswerving, untiring, unwavering,
   unwearied, unwearying, unwinking, unyielding, urgent,
   utterly attentive, weariless, wheedling