It is an adverb and the adjective is 'seamless' ,which could mean: 1- literally without seams, those lines of conjunctions formed by sewing together 2 pieces of material as in a seamless garment. 2- metaphorically it means perfectly consistent and coherent, continuous and flowing as in a seamless performance or a seamless plot of a story.

it means as to be seamless; without obvious joins or breaks; smoothly cowboy..