Strenuously, vigorously, as in She pulled on the rope with all her might and main. This expression is redundant, since the noun main also means “strength” or “power.” It survives only in this phrase, which may also be dying out. [Late 1200s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.