Douglas Harper's Etymology Dictionary
put-put
indicating the sound of a muffled internal combustion engine, 1904, imitative.
WordNet
put-put
n. a small gasoline engine (as on motor boat)
[also: put-putting, put-putted]
Usage examples of "put-put".
He diagrammed a sail in the air and made a put-put noise for the power.
He sat inside the deep blue room, staring out the windows at the vaporetti put-putting past, their running lights winking and darting like fantastic fireflies.