The Collaborative International Dictionary of English v.0.48:

Gratis \Gra"tis\ (gr[a^]"t[i^]s or gr[=a]"t[i^]s), adv. [L.,
   contr. fr. gratiis out of favor or kindness, without
   recompense, for nothing, fr. gratia favor. See Grace.]
   For nothing; without fee or recompense; freely; gratuitously.
   [1913 Webster]