A fair whack meaning a just portion or share is British slang and dates from the late 18th/early 19th century, presumably from having whacked or cut something into portions. ‘Top whack’ meaning highest rate of pay dates from the same period. In or out of whack means in or out of alignment and is American from the late 19th century, from knocking or whacking something out of kilter. To have a whack at something means to have a try and again is American from the late 19th century. Whacked meaning exhausted or tired is British informal from c. 1960. Whack meaning to kill is American slang from the 1980s and is simply a version of earlier slang, hit, meaning to kill that dates from the 1950s.