The word 'sputum' is a direct borrowing from Latin, where it is the neuter past participle of the verb 'spuere', meaning 'to spit'. The Latin verb itself can be traced back to the Proto-Indo-European root sp(y)eu̯-/sp(y)ū-, which carried the core meaning of 'to spit, to spew'. This root is also the ancestor of various English words like 'spew' and 'spit'. Thus, 'sputum' retains its original, literal meaning from Latin, referring specifically to matter that is spat out, now with a specialized medical connotation.

The word is a simplex loanword from Latin. It does not contain separable morphemes in its modern English spelling.