Source : Webster's Revised Unabridged Dictionary (1913)

Triweekly \Tri"week`ly\, a. [Pref. tri- + weekly.]
   Occurring or appearing three times a week; thriceweekly; as,
   a triweekly newspaper. -- adv. Three times a week. -- n. A
   triweekly publication.

   Note: This is a convenient word, but is not legitimately
         formed. It should mean occurring once in three weeks,
         as triennial means once in three years. Cf. {Biweekly}.