The Rice University Neologisms Database

To fall downwards in an undignified manner, usually as acted on by gravity. This word probably came from an onomatopoeic description of someone falling heavily. Blending and clipping is used: the ‘Fl-‘ is clipped from fly, flap, flutter (notion of being airborne) and is blended with ‘-ump’ clipped from ‘slump' (collapsing into a heap, slouching downwards). Flump is a defined word in an online dictionary (www.thefreedictionary.com); their definition is ‘To move or fall heavily with a dull sound’. The original source is unknown.