Only without one's agreement, only if one cannot prevent it. For example, Is he taking a second job?—Not if his wife can help it, or He's not riding on the back of that motorcycle, not if I can help it. This idiom uses help in the sense of “prevent” or “cause to be otherwise.” [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.