invalid (adj.1)
"not strong, infirm," 1640s, from Latin invalidus "not strong, infirm, weak, feeble," from in- "not" (see in- (1)) + validus "strong" (see valid). Specific meaning "infirm from sickness, disease, or injury" is from 1640s.
invalid (adj.2)
"of no legal force," 1630s, from special use of Latin invalidus (see invalid (adj.1)).
invalid (n.)
1709, originally of disabled military men, from invalid (adj.1). Invalides is short for French Hôtel des Invalides, home for old and disabled soldiers in the 7th arrondissement of Paris.