unbelievably; incredibly; to an unbelievable extent
Used adverbially to emphasize an extreme degree, often with adjectives or verbs. Literally 'to the extent that it's impossible to exist/be true'.
He runs unbelievably fast.
That movie was incredibly interesting.
Kanji spelling using 得; less common but still encountered.
Variant with 程; rare.
Variant with 有り and 程; rare.
From あり得ない (arienai, 'impossible, can't exist') + ほど (hodo, 'extent, degree'). The phrase is a fixed adverbial expression.