'unentertaining' is formed in English from the negative prefix 'un-' (from Old English 'un-' meaning 'not') + 'entertaining' (the present participle of 'entertain'). 'Entertain' ultimately comes from Latin via Old French: Old French 'entretenir' (to hold among, keep) from Latin elements where 'inter-' meant 'among' and 'tenēre' (tenere) meant 'to hold'.

'Entertain' developed from Latin-rooted Old French 'entretenir' and Middle English forms (such as 'enterten' / 'entertainen'), becoming modern English 'entertain'. Adding the participle suffix '-ing' produced 'entertaining', and the prefix 'un-' was attached to create 'unentertaining'.

Originally, forms of 'entertain' carried senses of 'hold', 'maintain' or 'receive and keep' (from keeping/holding), but over time the meaning shifted toward 'amuse' or 'provide enjoyment'; 'unentertaining' accordingly came to mean 'not amusing' or 'boring'.