In grammar and rhetoric, the separation of the parts of a
compound word, now generally done for humorous effect
by Teqcknologiqck June 20, 2003
The art of breaking one word into two parts, inserting a different word (often an expletive) in between those two parts, hence forth turning two different words into one in order to give extra emphasis

from Greek, a cutting, from temnein, to cut (see tem- in Indo-European roots)
Examples of tmesis are found in the following, well-known quotes:

La-dee-freakin-da, a less offensive alternative, was made famous via Chris Farley on SNL.

Wel-diddly-elcome, one of many ri-goddamn-diculous phrases Ned Flanders often drops on the Simpsons.

Other famous phrases include:
Fan-fucking-tastic (The Departed)
Ala-fucking-bama (My Cousin Vinny)
Viet-fucking-nam! (Forrest Gump)
I'm not o-fucking-kay (My Chemical Romance)
That makes him a lia-fucking-bility (Boondock Saints), and
A whole 'nother' (insertion of "whole" into "another," rather than "a whole new")
by Erkworm November 26, 2007