intuitive
reflexively
unplanned
unthoughted
involuntarily
on the hop
insensibly
spur-of-the-moment
spontaneously
automatism
unconscious
extemporaneously
abrupt
off the top of one's head
on impulse
nonspontaneous
unceremoniously
unspontaneously
blurt out
impulse buy
autoactive
ultroneously
unstudiedly
rock up
unprophetically
instantaneously
unceremonially
unperceivingly
senseless
nonmomentary
automaticism
unhesitantly
semispontaneous
unheededly
impulse purchase
carelessly
unblinkingly
out of the blue
insensate
unheedingly
instinctive
autonomic
intentionless
unthinkingly
unthinking
thoughtlessly
like a shot
unimaginatively
under the radar
in a flash