Etymology
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scream (n.)

c. 1500, "a sharp, piercing sound or cry," expressive of pain, alarm, etc., from scream (v.).

And (as they say) lamentings heard i' th' Ayre; Strange Schreemes of Death. ["Macbeth," II.iii.61]

That spelling probably reflects "sk-" as spelled in words from Latin (such as school; see sch); his early editions also have schreene for screen. The slang meaning "something very great, excellent, or exciting," especially "something that evokes a cry of laughter" is by 1888; screamer in this sense is from 1831.

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*gal- 
Proto-Indo-European root meaning "to call, shout."

It forms all or part of: call; clatter; Gallic; gallinaceous; gallium; glasnost; Glagolitic.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit garhati "bewail, criticize;" Latin gallus "cock;" Old English ceallian "to shout, utter in a loud voice," Old Norse kalla "to cry loudly," Dutch kallen "to talk, chatter;" German Klage "complaint, grievance, lament, accusation;" Old English clacu "affront;" Old Church Slavonic glasu "voice," glagolu "word;" Welsh galw "call."
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*wekw- 
Proto-Indo-European root meaning "to speak."

It forms all or part of: advocate; avocation; calliope; convocation; epic; equivocal; equivocation; evoke; invoke; provoke; revoke; univocal; vocabulary; vocal; vocation; vocative; vociferate; vociferous; voice; vouch; vox; vowel.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vakti "speaks, says," vacas- "word;" Avestan vac- "speak, say;" Greek eipon (aorist) "spoke, said," epos "word;" Latin vocare "to call," vox "voice, sound, utterance, language, word;" Old Prussian wackis "cry;" German er-wähnen "to mention."
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*kele- (2)
*kelə-, Proto-Indo-European root meaning "to shout." Perhaps imitative.

It forms all or part of: acclaim; acclamation; Aufklarung; calendar; chiaroscuro; claim; Claire; clairvoyance; clairvoyant; clamor; Clara; claret; clarify; clarinet; clarion; clarity; class; clear; cledonism; conciliate; conciliation; council; declaim; declare; disclaim; ecclesiastic; eclair; exclaim; glair; hale (v.); halyard; intercalate; haul; keelhaul; low (v.); nomenclature; paraclete; proclaim; reclaim; reconcile.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usakala "cock," literally "dawn-calling;" Latin calare "to announce solemnly, call out," clamare "to cry out, shout, proclaim;" Middle Irish cailech "cock;" Greek kalein "to call," kelados "noise," kledon "report, fame;" Old High German halan "to call;" Old English hlowan "to low, make a noise like a cow;" Lithuanian kalba "language."
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