'speech-capable' is a compound formed from 'speech' and 'capable'. 'speech' originates from Old English 'spæc'/'spræc' meaning 'spoken language' or 'talk', and 'capable' originates from French 'capable', ultimately from Latin 'capax' (root 'cap-' / 'capere' meaning 'to take' or notionally 'able to hold').

'speech' developed from Old English 'spæc'/'spræc' into Middle English 'speche' and modern English 'speech'. 'capable' passed into English via Old French and Middle English from Latin 'capax', yielding the modern adjective 'capable'; the compound 'speech-capable' is a modern productive formation combining these two elements.

Originally 'speech' referred specifically to 'spoken words' and 'capable' had senses tied to 'able to hold' or 'able', but over time 'capable' generalized to mean 'able to do something'; together the compound now means 'able to speak' or 'equipped for speech'.