What it means
をいいことに means “taking advantage of” a situation or fact, often by using it as a pretext for an inappropriate, excessive, or self-serving action. It commonly describes behavior viewed critically by the speaker. It attaches directly to a noun, or to a plain predicate followed by の; with a na-adjective, な comes before の. The surrounding sentence can be made familiar or polite without changing the grammar form itself.
Usage note
をいいことに usually carries a negative or reproachful nuance. Do not use it as a neutral equivalent of simply making good use of a favorable opportunity; constructions such as を利用して are more neutral.
How to use it
部長の不在をいいことに、社員たちは勤務時間中も私用の動画を見続けました。
Taking advantage of the department manager's absence, the employees continued watching personal videos even during working hours.
彼は丈夫なのをいいことに、毎晩ほとんど寝ずに働きました。
Taking advantage of being physically strong, he worked almost without sleeping every night.
彼は忙しくないのをいいことに、同僚の仕事まで勝手に引き受けました。
Taking advantage of not being busy, he took on even his colleague's work without permission.
両親が旅行に出ているのをいいことに、娘は友人たちを自宅に長時間招きました。
Taking advantage of the fact that her parents were away on a trip, the daughter invited her friends to her home for a long time.
部長の不在をいいことに、社員たちは勤務時間中も私用の動画を見続けた。
Taking advantage of the department manager's absence, the employees continued watching personal videos even during working hours.
彼は丈夫なのをいいことに、毎晩ほとんど寝ずに働いた。
Taking advantage of being physically strong, he worked almost without sleeping every night.
彼は忙しくないのをいいことに、同僚の仕事まで勝手に引き受けた。
Taking advantage of not being busy, he took on even his colleague's work without permission.
両親が旅行に出ているのをいいことに、娘は友人たちを自宅に長時間招いた。
Taking advantage of the fact that her parents were away on a trip, the daughter invited her friends to her home for a long time.