Wiktionary
thwap
interj. The sound of a heavy smack. vb. To make, or cause to make, a heavy smacking sound.
Usage examples of "thwap".
I felt pellets hit and heard them thwap through the plastic of my windbreaker, but the Kevlar softened the blow of the four or five that hit me.
Etta thwaps the pillows with her hands and the twin impressions of our heads vanish.