轻率

He did it in a rash moment.

It is too hasty to conclude that way.

give an imprudent promise; make rash promises

rush/jump to a conclusion; draw a hasty conclusion

make one's choice without due reflection

imprudently resolve/decide to do sth

deal with sth with levity; treat sth with levity

act indiscreetly/rashly

be indiscreet in choosing one's friends

speak out of turn

indiscreet/flippant behaviour