Green’s Dictionary of Slang

yap v.²

to pay back; thus yap-poo, yap-pu, pay up.

1859		Hotten Dict. of Modern Sl. etc. 131: YAP-POO, pay up.
1864, 1867, 1870		Hotten Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1891		Exeter & Plymouth Gaz. 15 Oct. 6/4: If I don’t have a cuta, I’ll cut my lucky of the whole concern. If you don’t yappo (hand over)!!
1895		Crowe Aus. Sl. Dict. 98: Yap, spelled backwards, is pay, etc.

In derivatives

yappy (adj.)

(over-) generous, exploitable.

1895

Crowe Aus. Sl. Dict. 98: Yappy, over generous, soft, foolish.