Present participle of retain, meaning to keep possession of something or to hold back from movement or escape.
From Latin retinere, combining re- ('back') and tenere ('to hold'). Literally means 'to hold back'. The word entered English via Old French retenir in the 14th century, initially used in legal contexts about holding property.
A 'retaining wall' perfectly demonstrates the word's literal meaning - it holds back earth or water from moving where you don't want it to go. The legal phrase 'retaining a lawyer' uses the same concept: you're holding onto their services so they can't work for the opposition. It's always about keeping something in place.
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