Uncertain; usually regarded as a back-formation from gibberish (see gibberish for more).
Meanings
To jabber, talk rapidly and unintelligibly or incoherently.
A little ere the mightiest Julius fell, / The graves stood tenantless and the sheeted dead / Did squeak and gibber in the Roman streets – […] c. 1599–1602 (date written), William Shakespeare, “The Tragedie of Hamlet, Prince of Denmarke”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act I, scene i]:
(Australia) A stone or rock, of chalcedony or similar mineral, found strewn over arid regions of inland Australia; a gibber stone.
(Australia, colloquial) Any small rock or stone, especially one used for throwing.
“Well, all I can say is that if yer don't take yer dial outer the road I'll bloomin' well take an' bounce a gibber off yer crust.” 1918, Norman Lindsay, The Magic Pudding, page 90:
(Australia, obsolete) A large boulder or rocky outcrop; also, an overhanging rock formation.
Etymology tree
English gib
Proto-Indo-European *-yósder.
Proto-Italic *-āzios
Latin -āriusnom.
Latin -āriusbor.
Proto-Germanic *-ārijaz
Proto-West Germanic *-ārī
Old English -ere
Middle English -ere
English -er
English gibber
From gib + -er.
Meanings
A balky horse.
1831-1850, William Youatt, On the Structure and the Diseases of the Horse
A hasty and passionate breaker will often make a really goodtempered young horse an inveterate gibber