What is the relationship between cubō and cumbō?

Question

Various prefixed verbs, such as recumbō "lie back" and succumbō "collapse", seem to point at a basic form *cumbō (-ere), meaning something like "lie down".

However, as far as I can tell, there is no such verb. Instead, we have cubō (-āre) with that same meaning, and a handful of derivations from this, such as accubō "lie at" and incubō "settle onto (v); nightmare (n)".

These two verbs certainly look similar, and my first instinct is that cumbō is just cubō with a Proto-Indo-European nasal infix (cmw mentions Greek λαμβάνω vs λάβον). But I haven't been able to find good etymological information online.

What is the origin of these two similar words? Do we know what happened to unprefixed cumbō that didn't happen to its prefixed forms?

Answer 1

Ernout-Millet's Dictionnaire supports my (and your) original thinking:

cumbo -is (le perfectum est le même que dans cubo, cf. le cas de sedeo, sido: sedi, et de sto, sisto: steti): type à infixe nasal et à voyelle thématique, marquant le fait que l'action s'accomplit, attesté seulement dans le composés, qui correspondent aux composés en -cubo, pour marquer l'aspect "déterminé" (procès arrivant à son terme).

And De Vaan's Proto-Italic reconstruction:

PIt. *kumbe/o- 'to lie down', *kubaje/o- 'be lying'

The Italic cognates De Vaan provides also lack the nasal infix, so my guess is that it disappeared early, perhaps displaced first by one of the prefixed verbs.

I'm sure you have both E-M and De Vaan, but I thought it is worth noting anyway, and perhaps someone else will fill in the rest of the information.