What is 50 40' or fight?

It's actually "Fifty-Four Forty or Fight" (or, in your notation, "54°40′ or Fight"), and it was the slogan of the expansionist-minded Democrats in the mid-1840s. The slogan refers to a particular line of latitude which defined the northern-most extent of the Oregon Country, a territory which was disputed and claimed jointly by both the United States and Great Britain. The territory under dispute included all of the modern states of Oregon, Washington, Idaho, and parts of Montana, Wyoming, and the Canadian province of British Columbia. Various proposals to resolve the joint claims of sovereignty were proposed, but proponents of "Manifest Destiny" thought that the United States should resolve the conflict with Great Britain by insisting on taking the entire territory (all the way North to the 54'40" line) and if necessary to fight (go to war) to do so. President James K. Polk ultimately resolved the dispute peacefully in the Oregon Treaty of 1846, by extending the border between the United States and British North America along the 49th parallel. The phrase "Fifty-Four Forty or Fight" has been attributed to William Allen, who was a Senator from Ohio at the time.