'Reminisce' is a back-formation from 'reminiscence' — from Latin 're-' (again) + 'mens' (mind). Re-minding.
To indulge in enjoyable recollection of past events; to talk or think about pleasant memories.
Back-formation from 'reminiscence,' from Late Latin 'reminīscentia' (the act of remembering), from Latin 'reminīscī' (to remember, to call to mind), from 're-' (again, back) + a root related to 'mēns' (mind) and 'meminisse' (to have remembered, to bear in mind). The Latin 'meminisse' is a perfect-form verb with no present tense, meaning 'to remember' is already complete — you have-remembered rather than are-remembering. The PIE root *men- (to think, to have in mind) is one of the most productive cognitive roots in the family, generating Sanskrit 'manas' (mind, thought, the thinking faculty in Vedic philosophy), Greek 'menos' (spirit, force, intention), Latin 'mens' (mind), and English 'mind' itself via Proto-Germanic *gamundiz. The prefix 're-' reinforces the backward-glancing quality — 'reminisce' is etymologically to
The verb 'reminisce' did not exist until 1829 — it was back-formed from the much older noun 'reminiscence' (16th century). This means English speakers had the noun for over 200 years before anyone thought to create a verb for the act of reminiscing. The same pattern created 'edit' from 'editor' and 'donate' from 'donation.'