look up any word, like the eiffel tower:
 
1.
immediately, right away
Open that letter forthwith! It's important!
by Waffles November 12, 2004
 
2.
at leisure; so as not to disrupt; in due time
He attended his duties forthwith; which is to say he spent the afternoon smelling the grass, contemplating what time the following week would best be filled with said activities.
by hermesius February 10, 2011
 
3.
Something you shout out after saying something to emphasize it. Usually meant to be funny.
Fred: Hand me the nachos Steve.
Steve: You get 'em, they're like two feet away from you.
Fred: FORTHWITH!
by Wesley Durrance September 19, 2005