big-time (adj, as used by mav in bigtime fireworks?) is in W3 <her presentation was definitely big-time>
it is undated; not much of a stretch to use it as an adverb (re-introducing... bigtime?).

AHD: adv : To an extreme degree; very much: “Sales are expanding, big-time.” [no date here either]

all of which is to say that it's been around long enough to be lexicographed.