The title comes from the Robert Burns poem "To a Mouse" , written in 1785:
The best laid schemes o' Mice an' Men,
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!
Translation to modern English: The best made plans of mice and men often go astray, and leave us nothing but pain and grief, rather than the joy we thought we would get from them.
I've always loved the title, as it tells a lot about the novel. A reader who is familiar with the Burns poem would know even before they started reading that this story is not going to come out well for some of the characters. Steinbeck's title here is a nice example of foreshadowing, and a demonstration of how important the choice of a title can be.