very bad: All drugs are bad, but crack is wack.
Origin of wack1
First recorded in 1935–40; perhaps back formation from wacky
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Liverpool and Midland English dialect friend; pal: used chiefly as a term of address
Word Origin for wack
perhaps from dialect wack or whack to share out, hence one who shares, a friend
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
"crazy person," 1938, back-formation from wacky. Adjective in slang sense of "worthless, stupid," is attested from late 1990s.
Online Etymology Dictionary, © 2010 Douglas Harper