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行為だ

is an act

→ 行為
phrase Uncommon #13,293
is an act 行為 means “act,” “action,” or “conduct,” often in formal or legal contexts, especially when judging an action as good, bad, illegal, or dangerous. だ means “is.”
JA: そんな危険な行為だとは思わなかった。
EN: I didn't think it was such a dangerous act.

自殺行為だよ

That’s suicidal.

— どん底作家の人生に幸あれ! (2019)

それはまさに侵略行為だ。

That is nothing short of an act of aggression.

捕虜を虐殺したのは残忍な行為だ。

Massacring prisoners of war was a brutal act.

自殺は絶望的な行為だ。

Suicide is an act of despair.

単独でその崖を登るのは大胆な行為だ。

Climbing that cliff alone is a bold undertaking.

降りるのは自殺行為だ

Getting off would be suicide.

— プラットフォーム (2019)

それは愚かな行為だ。

That is a foolish act.