A shortened term combining tomorrow and morning, usually used by illiterate students.
Tyrese: "Yo, Miss Smith, can I turn in this paper tomornin'?"
Miss Smith: "Sure, Tyrese. Whatever. Just don't shoot me."
The wonderful time that comes between midnight and 6am, it's after midnight so not technically night, but it's before daylight, so not technically morning. Also it's technically tomorrow, hence: to-mornin'
By the time they finished drinking and dancing at all of the local bars, it was tomornin'.