Functus officio ordinarily means a task performed; having fulfilled the function, discharged the office, or accomplished the purpose, and therefore of no further force or authority. (See Black’s Law Dictionary (6ed) page 673.) The latinism means in practice the idea that the specific duties and functions that an officer was legally empowered and charged to perform have now been wholly accomplished and thus the officer has no further authority or legal competence based on the original commission. This is because the thing which originally had life becomes dead or moribund after the performance of the duty or function by the authority. In our context, a Judge who has decided a question brought before him is functus officio, and cannot review his decision. (See also Sanusi v Ayoola (1992) 9 NWLR (Part 265) 275; Onwuchekwa v CCB (1991) 5 NWLR (Part 603) 409; Anyaegbunam v Attorney-General of Anambra State (2001) 6 NWLR (Part 710) 532; INEC v Nnaji (2004) 16 NWLR (Part 900) 473).
— Niki Tobi, JSC. Buhari v. INEC (2008) – SC 51/2008