Etymology
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inactivity (n.)

"want of action or exertion, sluggishness," 1640s, from in- (1) "not, opposite of" + activity. Phrase masterly inactivity attested by 1791.

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inertness (n.)
"inactivity; fact of being inert," 1660s, from inert + -ness.
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inactive (adj.)

"not active or acting," 1640s, from in- (1) "not, opposite of" + active. Perhaps a back-formation from inactivity.

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inertia (n.)
1713, "that property of matter by virtue of which it retains its state of rest or of uniform rectilinear motion so long as no foreign cause changes that state" [Century Dictionary], introduced as a term in physics 17c. by German astronomer and physician Johann Kepler (1571-1630) as a special sense of Latin inertia "unskillfulness, ignorance; inactivity, idleness," from iners (genitive inertis) "unskilled; inactive" (see inert). Also sometimes vis inertia "force of inertia." Used in 1687 by Newton, writing in Modern Latin. The classical Latin sense of "apathy, passiveness, inactivity" is attested in English from 1822.
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negligence (n.)

"heedless disregard of duty, inactivity, indifference, habit of omitting to do things which ought to be done," mid-14c., necligence, from Old French negligence "negligence, sloth; injury, injustice" (12c.), and directly from Latin neclegentia, neglegentia "carelessness, heedlessness, neglect," from neglegentem (nominative neglegens) "heedless, careless, unconcerned," present participle of neglegere "to neglect" (see neglect (v.)).

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quiet (n.)

c. 1300, "freedom from disturbance or conflict; calm, stillness," from Old French quiete "rest, repose, tranquility" and directly from Latin quies (genitive quietis) "a lying still, rest, repose, peace" (from PIE root *kweie- "to rest, be quiet").

From late 14c. as "inactivity, rest, repose;" from c. 1400 as "absence of noise."

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site (n.)
"place or position occupied by something," especially with reference to environment, late 14c., from Anglo-French site, Old French site "place, site; position," and directly from Latin situs "a place, position, situation, location, station; idleness, sloth, inactivity; forgetfulness; the effects of neglect," from past participle of sinere "let, leave alone, permit," from PIE *si-tu-, from root *tkei- "to settle, dwell, be home."
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dolce far niente 

"pleasing inactivity, sweet idleness," 1814, from Italian, literally "sweet doing nothing." The Latin roots are dulcis "sweet" (see dulcet), facere "to make, do" (see factitious), and nec entem, literally "not a being."

This phrase, frequent enough in English literature, does not seem to occur in any Italian author of note. Howells says that he found it current among Neapolitan lazzaroni, but it is not included in any collection of Italian proverbial sayings. [Walsh]
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lethargy (n.)

late 14c., litarge, "state of prolonged torpor or inactivity, inertness of body or mind," from Medieval Latin litargia, from Late Latin lethargia, from Greek lēthargia "forgetfulness," from lēthargos "forgetful," apparently etymologically "inactive through forgetfulness," from lēthē "a forgetting, forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English. The Medieval Latin word also is the source of Old French litargie (Modern French léthargie), Spanish and Italian letargia.

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layoff (n.)
also lay-off, lay off; 1889, "rest, relaxation, respite;" from the verbal phrase; see lay (v.) + off (adv.). Via seasonal labor with periodic inactivity, it came to have a sense of "temporary release from employment," and by 1960s was being used somewhat euphemistically for permanent releases of masses of workers by employers. The verbal phrase lay off is attested from 1841 (colloquial) as "stop working, be idle" (intransitive); 1892 as "dismiss" (an employee); meaning "stop disturbing" is from 1908. Its oldest sense is "remove and lay aside, rid oneself of" (1590s).
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