资料来源 : Webster's Revised Unabridged Dictionary (1913)

Costless \Cost"less\ (k?st"l?s; 115), a.
   Costing nothing.

资料来源 : WordNet®

costless
     adj : costing nothing; "complimentary tickets" [syn: {complimentary},
            {free}, {gratis(p)}, {gratuitous}]