From Latin 'incredibilis' (not to be believed), from 'credere' (to believe) — expanded from 'unbelievable' to 'amazing.'
Impossible or very difficult to believe; extraordinarily good or great.
From Latin incrēdibilis (unbelievable, beyond belief), composed of in- (not) and crēdibilis (worthy of belief, believable), from crēdere (to believe, to trust, to entrust), from PIE *ḱred-dheh₁- (to place one heart — a compound of *ḱerd- meaning heart and *dheh₁- meaning to place or set). This PIE compound is one of the most psychologically rich roots in the language: to believe was to put your heart into something. PIE *ḱerd- gave Latin cor/cordis (heart), Greek kardía, and English heart (via Proto-Germanic *hertō). PIE *dheh₁- gave Latin facere (to do, to make) and English do, deed, and fact. The word crēdere thus encodes an ancient theory of belief as cardiac commitment rather than intellectual assent. English borrowed the crēd- family
The German cognate 'unglaublich' (incredible) is a calque — a word-for-word translation of the Latin structure: 'un-' (not) + 'glaub-' (believe, from Proto-Germanic *galaubjaną) + '-lich' (like, -ible). Both the Latin and the Germanic forms express the same idea of 'not believable,' constructed with the same three morphological building blocks. The semantic drift from 'unbelievable' to 'really good' happened independently in both languages, showing that hyperbolic praise follows