Meaning

noun a component or accessory added to something after it has been manufactured

noun the act of adding a component or accessory to something that did not have it when it was manufactured

verb provide with parts, devices, or equipment not available or in use at the time of the original manufacture

originThe verb is derived from retro- (prefix meaning ‘back; backward’) + fit (“to equip, supply”). The noun, which is first attested later than the verb, is either derived: * from the verb (see above); or * from retro- + fit (“act of fitting; conformity of elements one to another”).

Etymology from Wiktionary, CC BY-SA 3.0