Crummy, cruddy, crappy, or otherwise not very good.
This made-up term, intended as a spoof of teenage slang of the day, was first used the the 1947 RKO motion picture, "The Bachelor and the Bobby-Soxer" starring Cary Grant, Myrna Loy, and
Shirley Temple. In the opening scene, when a maid wakes teenager Susan Turner, played by Temple, she protests being woken, saying "But, Bessie, I feel
absolutely sklonklish." Moments later, when Bessie relates the
conversation to Susan's older sister, Judge Margaret Turner, played by Loy, the sister says, "Next time she tells you she feels sklonklish, you tell her not to be a clunk."