expression, adverb
without thinking of the consequences; recklessly; heedlessly
Used when someone acts impulsively without considering what will happen afterwards. Often has a negative connotation.
彼は後先考えずに会社を辞めてしまった。
He quit his job without thinking of the consequences.
後先考えずに発言すると、後で後悔するよ。
If you speak without thinking of the consequences, you'll regret it later.
向こう見ず is an adjective/noun meaning 'reckless' or 'daredevil', focusing on a person's character, while 後先考えず is an adverbial phrase describing the manner of acting without forethought.
A set phrase combining 後先 (あとさき, 'before and after' or 'consequences') with the negative continuative form of 考える (かんがえる, 'to think'), literally 'without thinking of before and after'.