From guile +‎ -less.

• IPA^(key): /ˈɡaɪl.ləs/, [ˈɡaɪɫ.ləs]

• Audio (Southern England):

  (file)

guileless (comparative more guileless, superlative most guileless)

1. Free from guile; honest but naive.
   • 1923 February 11, Mme. X. [pseudonym], “News of Chicago Society: Now That War’s Over, On with Full Dress, Men”, in The Chicago Sunday Tribune, final edition, volume LXXXII, number 6, Chicago, Ill., →ISSN, →OCLC, part 7, page 5, columns 6–7:

     To foreigners we often seem guileless and overchatty, an impression which is dispersed when they find that much of our friendliness is just the ebullition of the moment and does not carry with it any permanence of devotion.

• See also Thesaurus:naive