The 'de' in indefatigable

From indefatigable:

ADJECTIVE

(of a person or their efforts) persisting tirelessly. ‘an indefatigable defender of human rights’

Origin

Early 17th century: from French, or from Latin indefatigabilis, from in- ‘not’ + de- ‘away, completely’ + fatigare ‘wear out.'

Is the de- the same as in deactivate? The origin of deactivate is not given in that dictionary.

I'm wondering why the word is not simply "infatigable" or even "unfatigable." Why is de- there? What is its function?

OED places the etymological breakdown of indefatigable as this:

Latin indēfatīgābilis , < (in- prefix) + dēfatīgāre to wear out

In the case of in-, the prefix is functioning as you expected:

to express negation or privation

The prefix de-, which derives from Latin, can have several meanings. One of them is "undoing something," which is what makes "indefatigable" sound as though it means "not able to undo fatigue," or, in other words, stuck fatigued.

However, the Latin de- prefix has other meanings, primarily these:

• down as in depress

• off or away as in decline

• imposing indignity as in delude, deride, or deceive.

It's likely that in the case of Latin dēfatīgāre the de- prefix functions to mean down. Think of indefatigable as something akin to undepressed. Such a nonce word would not mean "pressed," but rather "not pressed downward." Similarly, indefatigable doesn't mean "fatigable" but rather "not able to be pulled down by fatigue."