The rare (and unglossed) headword is not a participle, but an abstract noun expressing quality (ie. "thrown-to-the-ground-ness").
[1] This cognate adjective χαμαιπετής , here in the masculine singular nominative, is simularly glossed in other lexica; evidently quoted from somewhere (perhaps Plato, Symposium 203D), and cf. under upsilon 35.