Origins
The word 'maintain' entered English in the thirteenth century from Old French 'maintenir,' meaning 'βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββto keep,' 'to uphold,' or 'to support.' The French word descended from Vulgar Latin *manΕ«tenΔre, a compound of 'manΕ«' (ablative of 'manus,' hand) and 'tenΔre' (to hold). The literal meaning is 'to hold in the hand' β to keep a firm grip on something so that it does not deteriorate, collapse, or slip away.
The 'hand' element in 'maintain' connects it to one of English's largest word families. Latin 'manus' (hand), from PIE *man- (hand), generated 'manual' (done by hand), 'manuscript' (written by hand), 'manufacture' (originally made by hand, from 'manu facere'), 'maneuver' (to work by hand, from French 'manoeuvre,' from Medieval Latin 'manΕ«operΔre'), 'manage' (to handle, from Italian 'maneggiare,' to handle especially horses), 'manner' (a way of handling things), 'manifest' (struck by hand, hence evident), 'mandate' (given into one's hand, hence an authoritative order), 'emancipate' (to take out of the hand β to free from another's control), and 'manipulate' (to handle, especially skillfully or deceptively).
The 'hold' element connects 'maintain' to the '-tain' family: 'sustain,' 'obtain,' 'pertain,' 'attain,' 'contain,' 'retain,' 'detain,' and 'entertain.' Among this family, 'maintain' is distinguished by its prefix: while other members use abstract spatial prefixes (sub-, ob-, per-, ad-), 'maintain' uses a concrete body-part prefix. It is the most tactile word in the group β to maintain is not just to hold but to hold with the hand, implying direct, physical, ongoing engagement.
Development
The word entered English with three senses that all survive. First, to keep something in a specified state or at a specified level: 'maintain order,' 'maintain speed,' 'maintain a temperature.' Second, to keep something in good condition through regular care: 'maintain a building,' 'maintain equipment,' 'maintain a garden.' Third, to state something firmly as true: 'she maintained her innocence,' 'he maintained that the report was accurate.' The third sense derives from the medieval legal usage where 'to maintain' meant 'to uphold' in argument or in court β to hold a position firmly in one's hand and not release it.
The noun 'maintenance' (from Old French 'maintenance') carries the first two senses but not the third. One speaks of building maintenance, vehicle maintenance, or maintenance of standards, but not maintenance of an opinion. In legal English, 'maintenance' once had a specific technical meaning: the offense of supporting litigation in which one has no legitimate interest β essentially, holding up someone else's lawsuit. This legal sense has largely fallen out of common usage.
The phrase 'cap of maintenance' (a ceremonial cap carried before the sovereign as a symbol of authority) preserves the medieval meaning of 'maintain' as 'to uphold authority.' The word 'mainstay' β a crucial support β combines 'main' (from French 'main,' hand, from Latin 'manus') with 'stay' (a support), creating a doubly emphatic image of something that holds things up.