The Collaborative International Dictionary of English v.0.48:

Cogent \Co"gent\, a. [L. cogens, p. pr. of cogere to drive
   together, to force; co- + agere to drive. See Agent, a.,
   and cf. Coact to force, Coagulate, p. a.]
   1. Compelling, in a physical sense; powerful. [Obs.]
      [1913 Webster]

            The cogent force of nature.           --Prior.
      [1913 Webster]

   2. Having the power to compel conviction or move the will;
      constraining; conclusive; forcible; powerful; not easily
      reasisted.
      [1913 Webster]

            No better nor more cogent reason.     --Dr. H. More.
      [1913 Webster]

            Proofs of the most cogent description. --Tyndall.
      [1913 Webster]

            The tongue whose strains were cogent as commands,
            Revered at home, and felt in foreign lands.
                                                  --Cowper.

   Syn: Forcible; powerful; potent; urgent; strong; persuasive;
        convincing; conclusive; influential.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "cogent":
   acute, admissible, advantageous, apperceptive, appercipient,
   argute, armipotent, astute, auspicious, authoritative, beneficial,
   benevolent, binding, bon, bonny, braw, bueno, capital, commendable,
   compelling, consequential, consistent, constraining, convincing,
   credible, discerning, dynamic, effective, elegant, energetic,
   estimable, excellent, expedient, fair, famous, farseeing,
   farsighted, favorable, fine, forceful, forcible, foreseeing,
   foresighted, forethoughted, forethoughtful, good, goodly, grand,
   healthy, helpful, high-potency, high-powered, high-pressure,
   high-tension, in force, in power, incisive, inducing, influential,
   irresistible, just, justifiable, justified, kind, laudable, lawful,
   legal, legitimate, logical, longheaded, longsighted, meaningful,
   mighty, mighty in battle, momentous, nice, noble, operative,
   penetrating, perceptive, percipient, perspicacious, perspicuous,
   persuasive, piercing, plausible, pleasant, potent, powerful,
   prepotent, profitable, provident, puissant, rational, reasonable,
   regal, royal, ruling, sagacious, sane, satisfactory, satisfying,
   self-consistent, sensible, significant, skillful, solid, sound,
   splendid, striking, strong, substantial, sufficient, telling,
   trenchant, understanding, useful, valid, very good, vigorous,
   virtuous, vital, weighty, well-argued, well-founded, well-grounded,
   wholesome