The term 'not slack' can be represented by four different crossword solutions: TENSE (5 letters), TAUT (4 letters), SLAVE (5 letters), and TENSELY (7 letters). TENSE is an adjective meaning 'marked by strain or tension, ' while TAUT is an adjective meaning 'tightly stretched.' SLAVE is a noun referring to a person who is owned and controlled by another, and TENSELY is an adverb meaning 'in a TENSE manner.' All of these words are related to the idea of being tight, strained, or under tension, which is the opposite of being slack.