をいいことにTaking advantage of a convenient circumstance
をいいことに says that someone exploits a real, convenient circumstance to act selfishly, excessively, or improperly. It usually expresses criticism, though the criticism can be light or playful.
The two-part logic
The first part names a real circumstance that makes the later behavior easier: someone is absent, nobody is watching, oversight is weak, permission has been given, or information is unavailable. The second part describes how someone takes advantage of that opening. If the later action is simply sensible or beneficial, this expression will normally sound too critical.
A typical use
子どもたちは親が見ていないのをいいことに、夜中までゲームをしていた。
Taking advantage of the fact that their parents were not watching, the children played games until late at night.
AnalyzeThe parents' lack of supervision is the real condition that enables the children's behavior.
Different ways to state the circumstance
The form changes with how the convenient circumstance is expressed, but the opportunistic relationship stays the same.
上司が出張中なのをいいことに、彼は毎日早く帰っていた。
He took advantage of his boss being away on a business trip and left early every day.
AnalyzeThe noun ending takes なの before the target expression.
家主の留守をいいことに、男は勝手に部屋へ入った。
Taking advantage of the landlord's absence, the man entered the room without permission.
Analyze留守 directly names the enabling circumstance, so no clause-ending の is needed.
家の車を自由に使ってよいと言われたことをいいことに、兄は毎晩遅くまで乗り回している。
My brother is taking advantage of being told that he may use the family car freely and drives it around until late every night.
AnalyzeThe permission is real, but the speaker views the brother's use of it as excessive.
A real opening versus a claimed excuse
をいいことに focuses on exploiting a circumstance that genuinely enables the later action. By contrast, にかこつけて presents something as a pretext or claimed reason. Ask whether the first part is an actual opening or an excuse being invoked.
More examples
彼は誰も帳簿を確認しないのをいいことに、売上の数字を書き換えていた。
He took advantage of the fact that nobody checked the accounts and altered the sales figures.
Analyze業者は担当者が新人なのをいいことに、契約にない作業まで押しつけようとした。
The contractor tried to take advantage of the person in charge being new by pushing work on them that was not included in the contract.
Analyze警備が手薄なのをいいことに、何者かが資材を持ち出した。
Someone took advantage of the weak security and removed building materials.
Analyze店が空いているのをいいことに、私たちは閉店近くまで長居してしまった。
Taking advantage of the place being empty, we ended up staying until nearly closing time.
Analyze父の留守をいいことに、子どもたちは居間を散らかし放題にした。
Taking advantage of their father's absence, the children made as much mess as they pleased in the living room.
Analyze規則に明記されていないことをいいことに、責任を逃れようとするのは卑怯だ。
It is cowardly to exploit the fact that something is not explicitly stated in the rules in order to evade responsibility.
Analyze