The headword -- aorist middle, third person singular, of ἀναμάσσω -- is presumably extracted from the quotation given.
The LSJ entry on this verb notes, in relation to its earliest appearances (in Homer and Herodotus), the idea that murder left a stain of pollution which could be erased by wiping off the sword on the head of the slain victim. Later instances, probably including the present one, are figurative rather than literal.
[1] Aelian fr. 21 Domingo-Forasté (21 Hercher); again at alpha 2253.