not if one can help it

not if one can help it

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]
See also: can, help, if, not, one