not a native letter in Old English; in Anglo-French words it represents the "ts" sound (as in Anglo-French fiz, from Latin filius, modern Fitz); from late 13c. it began to be used for the voiced "s" sound and had fully taken that role by 1400. For letter name, see zed.
Thou whoreson Zed, thou vnnecessary Letter. ["King Lear," II.ii.69]
Series of zs to represent a buzzing sound first attested 1852; zees "spell of sleep, a nap" is slang first recorded 1963, American English student slang.