Is 'opportunistic' an adjective or a noun?

That he deployed it only on his last full day in office struck some observers as ***opportunistic***.

Question

Is 'opportunistic' an adjective or a noun?

That he deployed it only on his last full day in office struck some observers as ***opportunistic***.

Answer

... as opportunistic [behavior]

Adjective - opportunistic 
Adverb - opportunistically 
Noun - opportunist

Answer

It's an adjective. It's modifying the unmentioned or suggested "timing" or "moment" that he chose to do the deployment.

If you reword the sentence it might be clearer:
- Some observers thought that he chose an opportunistic moment to deploy it; his last day in the office.

- He chose an opportunistic time to do it.

Answer

It's usually used as an adjective, but could be used as an adverb (used to describe an action).