Antonym: quicken. Similar words: blacken, sicken, chicken, thicken, quicken, stricken, chicken out, packet. Meaning: ['slækən] 
v. 1. become slow or slower 2. make less active or fast 3. become looser or slack 4. make slack as by lessening tension or firmness.
v. 1. become slow or slower 2. make less active or fast 3. become looser or slack 4. make slack as by lessening tension or firmness. 61. Trade have slacken off.
More similar words: blacken, sicken, chicken, thicken, quicken, stricken, chicken out, packet, packed, jacket, lack, knacker, bracket, cracked, black, quackery, black out, lacking, linebacker, black hand, for lack of, blackmail, black list, blacklist, blackface, in the black, black sheep, blackberry, black horse, lacklustre.