Is it ensure, insure, or assure?
disappointing in quality or performance
Latin de- + salire 'to leap'
1580s, "skipping about," from Latin desultorius "hasty, casual, superficial," adjective form of desultor (n.) "a rider in the circus who jumps from one horse to another while they are in gallop," from desul-, stem of desilire "jump down," from de- "down" (see de-) + salire "to jump, leap" (see salient (adj.)). Sense of "irregular, without aim or method" is c.1740. Related: Desultorily; desultoriness.