It means to get something done by whatever means necessary - be they good or bad. It is sometimes suggested that 'by hook or by crook' derives from the custom in mediaeval England of allowing peasants to take from royal forests whatever deadwood they could pull down with a shepherd's crook or cut with a reaper's billhook. Another suggestion is that the phrase comes from the names of the villages of Hook Head and the nearby Crooke, in Waterford, Ireland. Hook Head and Crooke are on opposite sides of the Waterford channel and Cromwell (born 1599 - died 1658) is reputed to have said that Waterford would fall 'by Hook or by Crooke', i.e. by a landing of his army at one of those two places. A third theory is that the phrase derives from two learned judges, called Hooke and Crooke, who officiated during the reign of Charles I (born 1600 - died 1649) and who were called on to solve difficult legal cases. Hence, the cases would be resolved 'by Hooke or by Crooke'.

A hook is curved thing for grabbing. A crook is a staff with a hook at its end. In the old days of Merrie England, there are laws regarding how firewood can be collected from common lands. That is any branch that can reached either by a hook or a crook. Hence, the meaning to attain something by any means without regards how it is accomplished.

"by hook or by crook" means to reach one's goal using all possible or impossible ways regardless of morality or any principles, "stepping" over everything. e.g. he'll get this task done by hook or by crook.