Mαλθάσσεσθαι most frequent in the tragedians. Later writers seem to prefer μαλάσσεσθαι.
Beekes, ETYM. of μαλθακός: Formation like μαλακός; the two synonymous adjectives may have influenced each other. If μαλακός belongs with βλάξ, then μαλθακός must have taken its ending from μαλακός. Solmsen 1909:55 recognizes >μάλθη as the feminine of an original adj. *μαλθός, 'weak', which also left traces in μάλθων, etc.