It forms all or part of: caca; cachexia; caco-; cacoethes; cacophony; cucking stool; kakistocracy; poppycock.
It is the hypothetical source of/evidence for its existence is provided by: Greek kakke "human excrement," Latin cacare, Irish caccaim, Serbo-Croatian kakati, Armenian k'akor; Old English cac-hus "latrine."
Etymologists dispute whether the modern Germanic words (Dutch kakken, Danish kakke, German kacken), are native cognates or student slang borrowed from Latin cacare. Caca appears in Modern English in slang c. 1870, and could have been taken from any or several of the languages that used it.
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.