Etymology
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invalid (adj.1)
"not strong, infirm," also "infirm from sickness, disease, or injury", 1640s, from Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (from PIE root *wal- "to be strong"). With pronunciation from French invalide (16c.).
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invalid (n.)
"infirm or sickly person," 1709, originally of disabled military men, from invalid (adj.1). In Paris, Invalides is short for Hôtel des Invalides, home for old and disabled soldiers in the 7th arrondissement of Paris.
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invalid (adj.2)
"of no legal force," 1630s, from special use of Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (from PIE root *wal- "to be strong").
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invalidate (v.)

"destroy the strength or validity of, render of no force or effect," 1640s, from invalid (adj.2) + -ate (2). Related: Invalidated; invalidating.

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invalidity (n.)

"want of energy, force, or efficiency," 1540s, from French invalidité (16c.) or directly from Medieval Latin invaliditas "weakness, infirmity," from Latin invalidus "not strong, weak, feeble" (see invalid (adj.1)).

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invalidation (n.)

"act of rendering invalid," 1752, noun of action from invalidate (v.). Perhaps modeled on French invalidation (17c.).

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sickly (adj.)
late 14c., "ill, invalid, habitually ailing," from sick (adj.) + -ly (1). Meaning "causing sickness" in any sense is from c. 1600. Related: Sickliness.
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annulment (n.)
late 15c., "act of reducing to nothing;" see annul + -ment. Meaning "act of declaring invalid" (a statute, marriage, etc.) is recorded from 1660s; earlier in this sense was annulling (late 14c.).
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avoidance (n.)
late 14c., "action of emptying," from avoid + -ance. Sense of "action of dodging or shunning" is recorded from early 15c.; it also meant "action of making legally invalid," 1620s; "becoming vacant" (of an office, etc.), mid-15c.
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null (adj.)

"void of legal force, invalid," 1560s, from French nul, from Latin nullus "not any, none," from ne- "not, no" (from PIE root *ne- "not") + illus "any," diminutive of unus "one" (from PIE root *oi-no- "one, unique").

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