late 15c., "to place in or on (a vehicle)," from load (n.). Sense of "add to the weight of, put a load in or on" is from c. 1500; sense of "to charge a firearm" is from 1620s. Intransitive sense "put or take on a load or charge" is from 1720; of a vehicle, "to fill with passengers," from 1832. Of computer files or programs, by 1977. Related: Loaded; loaden (the old strong past participle, persisting till 18c. in poetry but now obsolete); loading.
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Harper, D. (n.d.). Etymology of loader. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/loader