Has to be Riddick. He was so bad, that RP had him try them for awhile underhanded.
But I also have a file with all of the stats in the 2000's.
Looking at that file, the worst single season (at least 1 attempt per game) have been, and I'm adding a (*) to non-post-players:
*Jasper - 36% on 25 attempts (injury limited to 20 games)
WCS - 37.2% on 86 attempts (but improved at least 10% each year)
Parker - 39.8% on 93 attempts
Lee - 40.3% on 62 attempts (he made only 32% the year before, but not enough attempts (25))
*Matthews - 41.2% on 34 attempts (not quite 1/gm, 36gms, but I still included since so bad)
*Whitney - 43.5% on 23 attempts (transferred after 18 games)
Dakari Johnson - 44.7% on 85 attempts (improved to 62.5% the next year)
*Briscoe - 46% on 124 attempts (how did a team with 3 guys making <50% still shoot 69% at the line?)
WCS - 48.2% on 83 attempts (improved to almost 62% the next year)
Then I looked up
Riddick's stats online, and he had at least 1 attempt per game all 4 years, making:
26.3%, 45.8%, 39.6%, 50.9%, so he made 41.3% across all 4 years.
I'm right-handed, but bet I could make 26% lefty.