'undeclared' originates from English, specifically formed by adding the negative prefix 'un-' to the past participle 'declared' (from the verb 'declare'). 'Declare' ultimately comes from Latin 'dēclārāre' ('dē-' meaning 'completely' + 'clārāre' related to 'clārus' meaning 'clear').

'declare' passed into English via Old French 'declarer' and Middle English 'declaren'; the modern adjective/participle 'declared' developed from these forms, and 'undeclared' was created by prefixing English 'un-' to mean 'not declared'.

Originally, 'declare' meant 'to make clear or show openly' (from the Latin sense). Over time, derived forms like 'declared' and 'undeclared' took on administrative/legal senses (for example, 'declared income' and 'undeclared income'), so the emphasis shifted toward formal announcement or reporting.