c. 1200, daungerous, "difficult to deal with, arrogant, severe" (the opposite of affable), from Anglo-French dangerous, Old French dangeros (12c., Modern French dangereux), from danger "power, power to harm, mastery, authority, control" (see danger).
In Chaucer, it can mean "hard to please; reluctant to give; overbearing." The modern sense of "involving danger, hazardous, unsafe, risky, liable to inflict injury or harm" is from c. 1400. Other words formerly used in this sense included dangersome (1560s), dangerful (1540s). Related: Dangerously.