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.
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.
c. 1600, "attentive in perceiving or taking notice, characterized by good powers of observation," also "attentive in observing what is prescribed or required" (a law, custom, etc.), from observe + -ant, or else from French observant, past participle of observer (see observance). In reference to Judaism, "strict in acting in accordance with precepts," from 1902. As a noun from late 15c. Related: Observantly; observantness.
It is the hypothetical source of/evidence for its existence is provided by: Avestan haurvaiti "to guard;" Latin servare "to guard, keep, watch;" Old Church Slavonic xraniti "to guard, protect;" Old High German gi-sarwi "armor, equipment," Old English searu "art, skill; wile, deceit."