'unassertive' originates from the prefix 'un-' meaning 'not' and the word 'assertive', which comes from the Latin 'assertus', the past participle of 'asserere', meaning 'to affirm'.

'assertive' changed from the Latin word 'assertus' and eventually became the modern English word 'assertive'. The prefix 'un-' was added to form 'unassertive'.

Initially, 'assertive' meant 'to affirm or declare', but over time, 'unassertive' evolved to mean 'lacking confidence or self-assurance'.