lieutenant [LME] A lieutenant is a ‘placeholder’ from French lieu ‘place’ and tenant ‘holding’. Locum tenens [M17th], now usually just locum [E20], is an exact translation in Latin. At first both expressions were used more generally of any person acting as a substitute, but now locum is usually just used of a doctor and lieutenant of a military or other officer [M16th]. Why the British pronounce the word as if it has an ‘f’ in it is a mystery, but the pronunciation has been in use since the earliest records.