madcap

adjective

1. wildly or heedlessly impulsive; reckless; rash: a madcap scheme.

noun

2. a madcap person.

adjective

1. impulsive, reckless, or lively

noun

2. an impulsive, reckless, or lively person

1580s, noun and adjective, from mad (adj.) + cap, used here figuratively for “head.” Related: Madcappery.