From Latin 'dubitare' (to waver), from 'dubius' (moving two ways) — doubt is, at root, the state of being split in two.
A feeling of uncertainty or lack of conviction; the state of being unsure about the truth or reliability of something.
From Old French 'doute' (modern 'doute'), from Latin 'dubitāre' (to waver, to hesitate, to be uncertain), from 'dubius' (wavering, uncertain), from PIE root *dwóh₁ (two). The deepest meaning is 'to be of two minds' — doubt is, at its etymological core, the state of being split in two, unable to choose between alternatives. The silent 'b' in modern English was inserted by Renaissance scholars
The silent 'b' in 'doubt' is a Renaissance spelling affectation. Middle English spelled the word 'doute' (matching its French source and its actual pronunciation). Sixteenth-century scholars re-inserted the 'b' to show the word's Latin origin 'dubitāre,' creating a spelling-pronunciation mismatch that has confused English learners for five centuries