perhaps from dialect wack or whack to share out, hence one who shares, a friend
wack in American English1
(wæk)
slang
noun
1.
wacko
adjective
2.
very bad
All drugs are bad, but crack is wack
3.
extreme; far-out
Word origin
[1935–40; perh. back formation from wacky]This word is first recorded in the period 1935–40. Other words that entered Englishat around the same time include: Arts and Crafts Movement, bingo, gangbuster, roadblock, unit trust