The Collaborative International Dictionary of English v.0.48:

Linguistic \Lin*guis"tic\ (l[i^][ng]*gw[i^]s"t[i^]k),
Linguistical \Lin*guis"tic*al\ (l[i^][ng]*gw[i^]s"t[i^]*kal), a.
   [Cf. F. linguistique.]
   Of or pertaining to language; relating to linguistics, or to
   the affinities of languages.
   [1913 Webster]

WordNet (r) 3.0 (2006):

linguistic
    adj 1: consisting of or related to language; "linguistic
           behavior"; "a linguistic atlas"; "lingual diversity"
           [syn: linguistic, lingual] [ant: nonlinguistic]
    2: of or relating to the scientific study of language;
       "linguistic theory"

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "linguistic":
   answering, articulated, communicating, communicational,
   communional, conversational, descriptive, enunciated,
   glottochronological, grammatic, graphemic, interacting,
   interactional, interactive, intercommunicational,
   intercommunicative, intercommunional, interresponsive,
   interrogative, interrogatory, lexicographic, lexicological,
   lexicostatistical, lingual, metalinguistic, morphological,
   morphophonemic, nuncupative, oral, parol, philological, phonemic,
   phonetic, phonological, pronounced, psycholinguistic, questioning,
   responsive, said, semantic, sounded, speech, spoken, structural,
   syntactic, telepathic, transmissional, unwritten, uttered, verbal,
   viva voce, vocal, vocalized, voiced, voiceful