'underminer' originates from English, specifically the verb 'undermine' plus the agentive suffix '-er', where 'under-' meant 'beneath' and the verb 'mine' originally meant 'to dig (a mine)'.

'underminer' changed from Middle English formations built on the verb 'undermine'; 'undermine' itself developed from Old English elements 'under' + a verb sense related to 'mine' (to dig), and through Middle English took the modern form 'undermine', with the agentive '-er' producing 'underminer'.

Initially, it meant 'one who digs under (a structure)' or 'a miner who undermines foundations', but over time it evolved to the broader figurative sense 'one who weakens or sabotages (authority, plans) from below'.