a dog, chiefly a small one
From a pet name, probably from an onomatopoeia for whistling. Compare dialectal fifi (interjection used to indicate that the seeker in a game of hide-and-seek may now start their search). See also Luxembourgish Fiffi below.
Ćwicz mówienie i zapamiętywanie " Fiffi " i wielu innych słów i zdań z niemiecki .