To fly off the handle is to lose your temper very suddenly. He flew off the handle, do not fly off the handle, flying off the handle again.
The suddenness is the whole of it, since the phrase describes an outburst with no warning. The image is an axe head coming loose and leaving the handle in the hand, which is an American phrase from the early nineteenth century.
1phrasefly off the handleTo lose your temper very suddenly.
“Don't mention his old car, or he'll fly off the handle.”
fly off the handle is an outburst.
From an axe head flying loose off its handle; an American phrase from the early nineteenth century.
From the image of an axe head becoming loose and suddenly flying free of its handle, a vivid metaphor for losing control. The phrase is an early 19th-century American idiom, with printed examples first appearing c. the 1810s–1830s.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms