Green’s Dictionary of Slang

(orig. US) a small vehicle or motor-boat.

1944		‘F. Bonnamy’ A Rope of Sand (1947) 129: He heard a putt-putt start and pull away into the night.
1949		Monteleone Criminal Sl. (rev. edn).
2007		q. in Wenner & Seymour Gonzo 156: [He] followed me night and day. Sometimes it would just be the two of us on a little putt-putt airplane going somewhere.