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(also off and on)
If something happens on and off during a period of time, it happens sometimes.
时断时续地;间歇地
I've had toothache on and off for a couple of months.
我的牙断断续续疼了几个月。
He studied for his degree on and off for 75 years.
他断断续续学习了75年的时间。
It rained on and off all day.
雨断断续续地下了一整天。