The Collaborative International Dictionary of English v.0.48:

Interminable \In*ter"mi*na*ble\, a. [L. interminabilis: cf. F.
   interminable. See Terminate.]
   Without termination; admitting no limit; boundless; endless;
   wearisomely protracted; as, interminable space or duration;
   interminable sufferings; an interminable sermon.
   [1913 Webster +PJC]

         That wild interminable waste of waves.   --Grainger.

   Syn: Boundless; endless; limitless; illimitable;
        immeasurable; infinite; unbounded; unlimited.
        [1913 Webster]

WordNet (r) 3.0 (2006):

interminable
    adj 1: tiresomely long; seemingly without end; "endless
           debates"; "an endless conversation"; "the wait seemed
           eternal"; "eternal quarreling"; "an interminable sermon"
           [syn: endless, eternal, interminable]

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "interminable":
   a mile long, ageless, all-comprehensive, all-inclusive,
   articulated, boundless, catenated, ceaseless, coeternal,
   concatenated, connected, constant, continual, continued,
   continuing, continuous, countless, cyclical, dateless, direct,
   drawn-out, endless, eternal, eterne, ever-being, ever-durable,
   ever-during, everlasting, everliving, exhaustless, extended,
   extending everywhere, extensive, far-flung, far-reaching, fargoing,
   featureless, gapless, illimitable, illimited, immeasurable,
   immediate, immemorial, immense, incalculable, incessant,
   incomprehensible, indestructible, inexhaustible, infinite,
   infinitely continuous, innumerable, interminate, joined, jointless,
   languishing, lasting, lengthened, lengthy, limitless, lingering,
   linked, long, long-continuing, long-drawn, long-drawn-out,
   long-pending, long-winded, longish, longsome, marathon,
   measureless, monotonous, never-ceasing, never-ending, no end of,
   nonstop, nonterminating, nonterminous, olamic, overlong,
   perdurable, perennial, periodic, permanent, perpetual, prolonged,
   protracted, recurrent, repetitive, round-the-clock, running,
   seamless, sempiternal, serried, sesquipedal, sesquipedalian,
   shoreless, smooth, spun-out, stable, steady, straight,
   stretched-out, sumless, tall, termless, timeless, twenty-four-hour,
   unbounded, unbroken, unceasing, uncircumscribed, undifferentiated,
   unending, unfathomable, uniform, unintermitted, unintermittent,
   unintermitting, uninterrupted, universal, unlimited, unmeasurable,
   unmeasured, unnumbered, unplumbed, unrelieved, unremitting,
   unstopped, untold, without bound, without end, without limit,
   without measure, without number