'overthinking' originates from English, specifically the prefix 'over-' (from Old English 'ofer') meaning 'above, beyond, excessive' combined with the verb 'think' (from Old English 'þencan') meaning 'to consider or ponder.'

'think' changed from Old English word 'þencan' to Middle English 'thinken' and eventually became the modern English 'think'; 'over' came from Old English 'ofer' via Middle English 'over'; the compound 'overthink' developed in modern English by adding 'over-' to 'think', and 'overthinking' is the present participle/gerund form used to denote the action or state.

Initially, 'think' meant 'to conceive or have in mind' and 'over-' meant 'excessively'; combined they initially and still mean 'to think excessively', though contemporary use often emphasizes unproductive or anxious rumination.