In an incautious manner; unwarily; heedlessly.

adverb. In an incautious manner; with a lack of caution.

adverb. without caution or prudence

"Bob, impressing upon him all my arrangements, in case of contingencies requiring an alteration in my original plan; for, as soon as we were fairly at work, everything would have to be done, as far as possible, in absolute silence, and I did not wish to leave any explanations for a moment when, perhaps, a single word incautiously uttered might lead to our betrayal."