Carries a skeptical tone - implies the denial is likely false or stubborn in the face of evidence. Very common in news reporting about scandals and criminal cases. Pairs with 面对证据, 面对质问.
memory
矢 means 'arrow', by extension 'to swear an oath' (as if by an arrow). 矢口 = 'with a sworn mouth', i.e. swearing with full force - so 矢口否认 is swearing up and down that it isn't so.
