The phrase "in lieu," or "in lieu of," means "instead" or "in place of." For example: "Tom agreed to file Susan's taxes for her in lieu of paying her in cash."
In American English, "in lieu" can be used in both social and legal contexts. However, the meaning is the same whether wedding guests are asked to make a charitable donation to "in lieu of gifts," or an employer offers a terminated employee payment "in lieu of notice."
The phrase "in lieu" (pronounced "loo") dates back to the 13th century and comes to English from the Latin word "locus" by way of Middle French. According to the Oxford English Dictionary, the word "lieu" is also used in modern French where it means "place" or "location." "Milieu," another French word from the same root, and with a similar meaning, has also been borrowed into English.
The Latin word "locus" is a common root word and can be seen in modern English words like "locate" and "locale." Even though it is directly borrowed from Latin, the legal term "in loco parentis," which refers to someone acting in place of a parent, came into use much more recently than "in lieu." The first known use of that phrase in English is from 1818.