see under off and on.
Examples from the Web for on again
The 24-year-old Buffalo native had a "date" that night, but she wouldn't tell her on-again, off-again boyfriend with whom.
I moved back to the city and my two on-again off-again guys, steeling myself for hopeless fix-ups, just in case love might strike.
Cale, absent from this reunion, has had an on-again off-again feud with Reed and was barely mentioned.