'Tremendous' is Latin for 'to be trembled at' — from 'tremere.' It weakened from terrifying to just 'very large.'
Very great in amount, scale, or intensity; extraordinarily large or impressive.
From Latin tremendus (that which must be trembled at, awesome, fearful, causing trembling), gerundive of tremere (to tremble, to quiver, to shake in fear), from PIE *trems- (to tremble, to quake), related to *trem- (to shake). The PIE root *trem- also gives Greek tremein (to tremble), Lithuanian tremu (I tremble), and Sanskrit tarasate (he trembles, hastens). The gerundive form tremendus implies an obligation — this is a thing that ought to cause trembling, something so awe-inspiring that trembling is the proper response. The word entered
'Tremendous' originally meant 'so terrifying it makes you tremble,' but semantic inflation turned it into a casual intensifier meaning 'really great.' The same process weakened 'awesome' (inspiring awe), 'terrific' (causing terror), and 'wonderful' (causing wonder) from the sublime to the merely positive.