Single Word Requests – Correct Term for Juxtaposition of Words

This is a rhetorical device known as antiphrasis.

antiphrasis
n
(Literature / Rhetoric) Rhetoric the use of a word in a sense opposite to its normal one, esp for ironic effect

An example of this would be Perdue Chicken's advertising tag line of a couple decades ago:

It takes a tough man to make a tender chicken.

Other examples:

"Come here, Tiny," he said to the fat man.
It was a cool 115 degrees in the shade.

Note that the irony is established by context, i.e., reference to another part of the sentence or to an obvious fact. In the examples above, note that "Tiny" and "cool" are used in deliberate contrast to information related later. This differs from simple irony, which is a statement whose intended meaning is the opposite of its literal one, in that it must play on that opposite within the statement. For example, in the "Tiny" example, if the speaker were simply to say to the fat man

"You look like a skeleton."

That would be an ironic statement but it would not be antiphrasis. Antiphrasis requires the play of "tiny" and "fat" for its effect.