Sentence:  ... there he was gallantly, thoughtlessly alive, to all appearance indestructible solely by the virtue of his few years and of his <WSD>unreflecting</WSD> audacity. 
Meaning of the word marked with <WSD> tags:  ("of a person") Not giving reflection or thought to one's actions, words, conclusions, etc.; ("of an action, statement, conclusion, etc.") done without reflection or thought.