'unfissured' is formed from the English negative prefix 'un-' (from Old English 'un-' meaning 'not') combined with 'fissured', which ultimately derives from Latin 'fissura' (from 'findere' meaning 'to split').

'fissura' in Latin passed into English via Old French/Middle English as 'fissure', producing the adjective 'fissured'; the modern formation 'unfissured' arose by prefixing 'un-' to that adjective.

Initially the components referred to 'not split' in a literal, physical sense; the modern meaning remains largely the same: 'not having fissures or cracks.'