expression, noun
good thing; nice thing
General expression for something positive, beneficial, or pleasant.
今日はいいことがあった。
Something good happened today.
運動は体にいいことだ。
Exercise is a good thing for your body.
expression, noun
good excuse; good opportunity; taking advantage of
Used in the pattern 〜をいいことに(して) meaning 'taking advantage of (a situation)' or 'using as a good excuse'. Often carries a negative nuance of exploiting a situation.
彼は忙しいのをいいことに、会議をサボった。
He took advantage of being busy and skipped the meeting.
親が留守なのをいいことに、友達を呼
expression, noun
Children's language or casual expression meaning 'That's a good idea' or 'Let's do that'. Often used as a suggestion.
いいこと考えた！
I've got a good idea!
公園に行くのはいいことだね。
Going to the park is a good idea, isn't it?
interjection
Interjection used to impress an idea or urge a response, often by women or in casual speech. Similar to 'Hey, guess what?' or 'You know what?'.
いいこと、明日のパーティーに来ない？
Hey, why don't you come to the party tomorrow?
いいこと、秘密を教えてあげる。
Listen, I'll tell you a secret.
Taking advantage of his parents being away, he invited friends over and had a party.
Combination of the adjective いい (good) and the nominalizer こと (thing). The interjection and children's usage developed from the idea of presenting a 'good thing' or 'good idea'.