See slacken in Webster's New World Roget's A-Z Thesaurus II
To reduce in tension, pressure, or rigidity:
ease, let up, loose, loosen, relax, slack, untighten. See tightenTo become or cause to become less active or intense:
abate, bate, die (away, down, off, or out), ease (off or up), ebb, fall, fall off, lapse, let up, moderate, remit, slack off, subside, wane. See increaseTo moderate or change a position or course of action as a result of pressure:
ease off, relent, soften, weaken, yield. Idiom: give way (or ground). See strongLearn more about slacken