Definition of "painfully"

The unsufficiency and uncandidness of his answer became painfully apparent in the dead silence of the room.

1906 January–October, Joseph Conrad, chapter IX, in The Secret Agent: A Simple Tale, London: Methuen & Co., […], published 1907; The Secret Agent: A Simple Tale (Collection of British Authors; 3995), copyright edition, Leipzig: Bernhard Tauchnitz, 1907, page 193