Etymology
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hollow (adj.)

c. 1200, adjective developed from Old English holh (n.) "hollow place, hole," from Proto-Germanic *hul-, from PIE root *kel- (1) "to cover, conceal, save." The figurative sense of "insincere" is attested from 1520s. Related: Hollowly. Spelling development followed that of fallow, sallow. Adverbial use in carry it hollow "take it completely" is first recorded 1660s, of unknown origin or connection. Hollow-eyed "having deep, sunken eyes" is attested from 1520s.

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hollow (v.)
late 14c., "to make hollow," holowen, from hollow (adj.). Related: Hollowed; hollowing. Old English had holian "to hollow out."
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hollow (n.)
"lowland, valley, basin," 1550s, probably a modern formation from hollow (adj.), which is from Old English holh (n.) "cave, den; internal cavity."
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hollowness (n.)
late 14c., "cave, cavern; internal empty space;" mid-15c., "condition of being hollow," from hollow (adj.) + -ness.
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hole (n.)

Old English hol (adj.) "hollow, concave;" as a noun, "hollow place; cave; orifice; perforation," from Proto-Germanic *hulan (source also of Old Saxon, Old Frisian, Old High German hol, Middle Dutch hool, Old Norse holr, German hohl "hollow," Gothic us-hulon "to hollow out"), from PIE root *kel- (1) "to cover, conceal, save." As an adjective, it has been displaced by hollow, which in Old English was only a noun, meaning "excavated habitation of certain wild animals."

As a contemptuous word for "small dingy lodging or abode" it is attested from 1610s. Meaning "a fix, scrape, mess" is from 1760. Obscene slang use for "vulva" is implied from mid-14c. Golfing hole-in-one is from 1914; as a verbal phrase from 1913. To need (something) like a hole in the head, applied to something useless or detrimental, first recorded 1944 in entertainment publications, probably a translation of a Yiddish expression such as ich darf es vi a loch in kop.

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*kel- (1)

Proto-Indo-European root meaning "to cover, conceal, save."

It forms all or part of: Anselm; apocalypse; Brussels; caliology; Calypso; calyx; ceiling; cell; cellar; cellular; cellulite; cellulitis; cilia; clandestine; cojones; coleoptera; color; conceal; eucalyptus; hall; hell; helm (n.2) "a helmet;" helmet; hold (n.2) "space in a ship below the lower deck;" hole; hollow; holster; housing (n.2) "ornamental covering;" hull (n.1) "seed covering;" kil-; kleptomania; occult; rathskeller; supercilious; Valhalla; William.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit cala "hut, house, hall;" Greek kalia "hut, nest," kalyptein "to cover," koleon, koleos "sheath," kelyphos "shell, husk;" Latin cella "small room, store room, hut," celare "to hide, conceal," clam "secret," clepere "to steal, listen secretly to;" Old Irish cuile "cellar," celim "hide," Middle Irish cul "defense, shelter;" Gothic hulistr "covering," Old English heolstor "lurking-hole, cave, covering," Gothic huljan "to cover over," hulundi "hole," hilms "helmet," halja "hell," Old English hol "cave," holu "husk, pod;" Old Prussian au-klipts "hidden;" Old Church Slavonic poklopu "cover, wrapping."

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excavate (v.)

"to hollow out, make hollow by digging or scooping, or by removing extraneous matter," 1590s, from Latin excavatus, past participle of excavare "to hollow out," from ex "out" (see ex-) + cavare "to hollow, hollow out," from cavus "cave" (from PIE root *keue- "to swell," also "vault, hole"). Related: Excavated; excavating.

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Glasgow 
city in Scotland, from Gaelic, literally "green hollow," from glas "green, verdant" + cau "hollow."
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concavity (n.)

c. 1400, "a concave surface," from Old French concavit "hollow, concavity" (14c.) or directly from Latin concavitatem (nominative concavitas), from Latin concavus "hollow, arched, vaulted, curved," from con-, here perhaps an intensive prefix (see con-), + cavus "hollow" (from PIE root *keue- "to swell," also "vault, hole"). From 1570s as "state of being concave."

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coelo- 

before vowels coel-, word-forming element in scientific compounds meaning "hollow," from Latinized form of Greek koilos "hollow," from PIE root *keue- "to swell," also "vault, hole."

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