Etymology
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observe (v.)

late 14c., observen, "to hold to (a manner of life or course of conduct), carry out the dictates of, attend to in practice, to keep, follow," from Old French observer, osserver "to observe, watch over, follow" (10c.), from Latin observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect." Sense of "watch, perceive, notice" is from 1560s, via the notion of "see and note omens." Meaning "to say by way of remark" is from c. 1600. Related: Observed; observing.

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observer (n.)

1550s, "one who keeps a rule, custom, etc.," agent noun from observe. Meaning "one who watches and takes notice" is from 1580s; this is the sense of the word in many newspaper names. Meaning "one who observes without participating" (at a meeting, conference, etc.) is by 1925.

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obsess (v.)

c. 1500, "to besiege" (a sense now obsolete), from Latin obsessus, past participle of obsidere "watch closely; besiege, occupy; stay, remain, abide" literally "sit opposite to," from ob "against" (see ob-) + sedere "to sit," from PIE root *sed- (1) "to sit." Of evil spirits, "to haunt," from 1530s. The psychological senseof "to haunt as a fixed idea" developed gradually from 1880s and emerged 20c. The 1895 Century Dictionary has only the two senses "to besiege" (marked obsolete) and "to attack, vex, or plague from without." Related: Obsessed; obsessing.

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obsessed (adj.)

mid-15c., obcessed, "tormented, obsessed," past-participle adjective from obsess. Originally especially "possessed" by a devil or fiend.

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obsession (n.)

1510s, "action of besieging" (a sense now obsolete), from French obsession and directly from Latin obsessionem (nominative obsessio) "siege, blockade, a blocking up," noun of action from past-participle stem of obsidere "to besiege" (see obsess). Later (c. 1600), "hostile action of an evil spirit" (like possession but without the spirit actually inhabiting the body). Transferred sense of "action of anything which engrosses the mind" is from 1670s. Psychological sense "idea or image that intrudes on the mind of a person against his will" is from 1901.

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obsessive (adj.)

"of or pertaining to obsession; liable to obsess," 1911, from obsess + -ive. As a noun, "person characterized by obsession," by 1966. Related: Obsessively. Obsessive-compulsive "combining (psychological) obsessions and compulsions" is attested from 1927.

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obsidian (n.)

"dark, hard, glass-like volcanic rock," 1650s, from Latin obsidianus, misprint of Obsianus (lapis) "(stone) of Obsius," name of a Roman alleged by Pliny to have found this rock in Ethiopia.

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obsolesce (v.)

"fall into disuse, grow obsolete," 1801, from Latin obsolescere "to grow old, wear out, lose value, become obsolete," inchoative of obsolere "fall into disuse" (see obsolete). Related: Obsolesced; obsolescing.

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obsolescence (n.)

"state or process of gradually falling into disuse, a becoming obsolete," 1809; see obsolescent + -ence. Phrase planned obsolescence was coined 1932, revived as a disparaging term 1950s.

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obsolescent (adj.)

"becoming obsolete, passing out of use," 1755, from Latin obsolescentum (nominative obsolescens), present participle of obsolescere "fall into disuse" (see obsolete).

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