The Collaborative International Dictionary of English v.0.48:

Amour \A*mour"\, n. [F., fr. L. amor love.]
   1. Love; affection. [Obs.]
      [1913 Webster]

   2. Love making; a love affair; usually, an unlawful
      connection in love; a love intrigue; an illicit love
      affair.
      [1913 Webster]

   In amours with, in love with. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

amour
    n 1: a usually secretive or illicit sexual relationship [syn:
         affair, affaire, intimacy, liaison, involvement,
         amour]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "amour":
   adulterous affair, adultery, affair, amorousness, cuckoldry,
   entanglement, eternal triangle, flirtation, forbidden love,
   hanky-panky, illicit love, infidelity, intimacy, intrigue, liaison,
   love, love affair, passion, relationship, romance, romantic tie,
   triangle, unfaithfulness