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Webster 1913 Edition

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Adj.

Not dreamed, or dreamed of; not th[GREEK]ught of; not imagined; – often followed by of.

Unpathed waters,

undreamed

shores.

Shakespeare

undreamt ‎(comparative more undreamt, superlative most undreamt)

Not dreamed; not dreamt.
In the end, he regretted only the words left unspoken and the dreams left undreamt.