Some sort of derivation from *déḱm̥(“ten”) (with Pre-IE *d lost or merged with *h₁ in the same way the initial dental obstruent was lost in the oblique cases of *dʰéǵʰōm(“earth”)), perhaps its ordinal number (*déḱm̥ + *-ó-). If so then this could come from some phrase, "tenth", whose substantive noun can only be conjectured.