c. 1300, maintenen, "to support, uphold, aid;" also "hold fast, keep in possession, preserve from capture or loss," from Anglo-French meintenir (Old French maintenir, 12c.) "keep (a wife), sustain; persevere in, practice continually," from Latin manu tenere "hold in the hand," from manu, ablative of manus "hand" (from PIE root *man- (2) "hand") + tenere "to hold," from PIE root *ten- "to stretch."
Sense of "hold in an existing state or condition, keep in existence or continuance" is from early 14c. Meaning "to carry on, keep up" is from mid-14c.; that of "to keep oneself, support" is from late 14c. Sense of "defend in speech, uphold by argument or assertion" is from mid-14c. Meaning "practice habitually" is from c. 1400. Sense of "furnish means for the subsistence or existence of" is from c. 1400. Related: Maintained; maintaining; maintains.