To a degree that is hard to believe — the adverb form of staggering, used to turn an adjective all the way up. Its normal position is directly in front of an adjective: staggeringly expensive, staggeringly rich, staggeringly beautiful, staggeringly stupid, staggeringly successful. Think of it as a stronger, more surprised-sounding replacement for 'extremely' or 'incredibly'. Two useful points about how it behaves. It goes with adjectives, not usually with verbs — you say 'staggeringly expensive', not 'it cost staggeringly'. And unlike some strong adverbs, it works with both good and bad qualities; the adjective decides the direction, staggeringly only supplies the size. It also pairs happily with numbers and comparisons: 'staggeringly high', 'staggeringly more than last year'. In register it is a written and educated-speech word — reviews, articles, opinion pieces, and people who read a lot. It is not slangy, but it is not neutral either; it always sounds like the writer is reacting, leaning in, asking you to notice. Because of that, one per paragraph is plenty. It modifies an adjective; the adjective itself modifies a noun.