Wiktionary
vb. 1 (context intransitive English) to slacken, make slack or slacker 2 (context intransitive English) to become less intense 3 (context transitive English) to do (something) less intensely
WordNet
Usage examples of "slacken off".
But toward late afternoon, the rain started to slacken off altogether and the clouds began to thin out, permitting small strips of blue to seep through the rolling grayness.
It had started to slacken off its involvement, and stand down the forces it had secretly brought up to near the Chelgrian sphere of influence and colonisation in case things went wrong, when, without any warning, it had all gone quite spectacularly wrong.
No matter where or when, no lieutenant had ever been known to slacken off on a middie.
I couldn't increase the pressure there but I had to maintain it because he was waiting for me to slacken off and lose the initiative and then he would move his right hand and fix on the target again and squeeze his index finger and send a 200-grain hollow-point projectile into my skull and through the soft grey convolutions of my brain at 1500 feet per second and I didn't want him to do that.
It moved quickly, the wind pushing the torrent forward in a steady line, a downpour that lasted a mercifully brief time before beginning to slacken off.
Frank swung the junk's nose around into the wind to lose headway and ordered the others to slacken off on the sheets.
The airport commandant admits that things do slacken off a bit when there's nothing scheduled.