Impossibly means to a degree beyond what can actually be done or exist. Impossibly small, impossibly high standards, impossibly young.
It is an exaggeration by design, since what is described plainly does exist. Impossibly young is the standard thing said about anybody in a job you expected an older person to hold.
1adverbimpossiblyTo a degree beyond what can actually be done or exist, as of a task no one could complete or a waist too slender to be real.
“The cliff face was impossibly steep, so we turned back.”
impossibly is a degree.
Middle English
impossible
English
impossible
English, today
impossibly
From impossible + -ly. impossible comes from Middle English impossible, equivalent to im- + possible.
Etymology from Wiktionary, CC BY-SA