'restain' is formed in Modern English by adding the prefix 're-' (from Latin 're-' meaning 'again') to the verb 'stain' (a Middle English verb meaning 'to discolor or dye').

'stain' comes from Middle English 'stainen' (to discolor, dye), influenced by Old French terms for dyeing; the modern verb 'restain' arose by productive use of the prefix 're-' in Modern English to indicate repetition, creating 're-' + 'stain'.

The element 'stain' originally meant 'to discolor or dye'; by adding 're-' the meaning specialized to 'to stain again' or 'to apply a new stain', which is the current sense of 'restain'.