mid-14c., "hazardous;" late 14c., "occurring by chance" (senses now obsolete), from Old French aventuros "chance, accidental, fortuitous;" of persons, "devoted to adventure" (Modern French aventureux), from aventure (see adventure (n.)). In English the sense of "rash, risk-taking" is from c. 1400, thence "daring, fond of adventure" (mid-15c.). Related: Adventurously; adventurousness.
The adventurous man incurs risks from love of the novel, the arduous, and the bold, trusting to escape through the use of his bodily and mental powers; he would measure himself against difficult things. When this spirit does not go so far as to deserve the name of rashness or foolhardiness, it is considered a manly trait. [Century Dictionary]