What is the meaning of Wah-wah?

Modifying the resonance of musical notes to produce the effect of a human voice.

The sad trombone sound, used to indicate failure or disappointment. May be used mockingly; also written wah wah and womp womp.

A wavering sound produced by means of a movable mute or by electronic means

to make a wah-wah sound

Source: wiktionary.org