gratis (adv.) Look up gratis at Dictionary.com
mid-15c., "for nothing, freely," from Latin gratis, contraction of gratiis "for thanks," hence, "without recompense, for nothing," ablative of gratiae "thanks," plural of gratia "favor" (from suffixed form of PIE root *gwere- (2) "to favor"). Meaning "free of charge" is 1540s.