Source : Webster's Revised Unabridged Dictionary (1913)

Gratis \Gra"tis\, 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.

Source : WordNet®

gratis
     adj : costing nothing; "complimentary tickets" [syn: {complimentary},
            {costless}, {free}, {gratis(p)}, {gratuitous}]
     adv : without payment; "I'll give you this gratis" [syn: {for free},
            {free of charge}]