If something is efficient, it's productive and inexpensive. Inefficient has the opposite meaning, which is clear from the "not" prefix in-.

What type of word is inefficient?

Not efficient; not producing the effect intended or desired; inefficacious; as, inefficient means or measures.

Does unacceptable have a prefix?

The adjective unacceptable adds the "not" prefix un- to acceptable, "suitable, adequate, or pleasing." We can trace the roots back to the Latin acceptare, "take or receive willingly."