5. Important; necessary. That which thou aright Believest so main to our success, i bring. (Milton) By main force, by mere force or sheer force; by violent effort; as, to subdue insurrection by main force. That Maine which by main force Warwick did win. (Shak) By main strength, by sheer strength; as, to lift a heavy weight by main strength. Main beam, the principal or true keel of a vessel, as distinguished from the false keel.