Etymology
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infirm (adj.)

late 14c., "weak, unsound" (of things), from Latin infirmus "weak, frail, feeble, not strong or firm" (figuratively "superstitious, pusillanimous, inconstant"), from in- "not, opposite of" (see in- (1)) + firmus "strong; stable," figuratively "constant, trusty" (from suffixed form of PIE root *dher- "to hold firmly, support" ). Of persons, "not strong, unhealthy," first recorded c. 1600. As a noun from 1711.

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infirmary (n.)
mid-15c., "sick bay in a monastery," formerly also enfermerie, also firmary, fermery, from Old French enfermerie "hospital" and directly from Medieval Latin infirmaria "a place for the infirm," from Latin infirmus "weak, frail," (see infirm). According to OED, the common name for a public hospital in 18c. England.
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infirmity (n.)

late 14c., infirmite, "disease, sickness; lack of capability, weakness," from Old French infirmité, enfermete "illness, sickness, disease; moral weakness," and directly from Latin infirmitatem (nominative infirmitas) "want of strength, weakness, feebleness," also "the weaker sex" [Lewis], noun of quality from infirmus "weak, frail" (see infirm). 

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*dher- 

Proto-Indo-European root meaning "to hold firmly, support." 

It forms all or part of: affirm; confirm; Darius; dharma; farm; fermata; firm (adj.); firm (n.); firmament; furl; infirm; infirmary; terra firma; throne.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dharmah "custom, statute, law," dharayati "holds;" Prakrit dharaṇa "a holding firm;" Iranian dāra‑ "holding;" Greek thronos "seat;" Latin firmus "strong, steadfast, enduring, stable;" Lithuanian diržnas "strong;" Welsh dir "hard," Breton dir "steel."

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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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mancinism (n.)

"left-handedness," 1890, from Italian mancinissmo, from mancino "infirm (in the hand)," from manco, from Latin mancus "maimed, infirm, crippled; lame-handed," from PIE *man-ko- "maimed in the hand," from PIE root *man- (2) "hand."

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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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decrepitude (n.)

"state of being broken down by infirmities," c. 1600, from French décrépitude (14c.), from Latin decrepitus "very old, infirm," from de "down" (see de-) + *crepitus, past participle of crepare "to crack, break" (see raven).

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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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debilitative (adj.)

"tending to render weak or infirm," 1680s, with -ive + Latin debilitat-, stem of debilis "lame, disabled, crippled," figuratively "weak, helpless," from de "from, away" (see de-) + -bilis "strength," from PIE root *bel-"strong" (see Bolshevik). 

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