[1690–1700; un-2 + thatch]This word is first recorded in the period 1690–1700. Other words that entered Englishat around the same time include: additive, dogwatch, migrate, momentum, syntheticun- is a prefix freely used in English to form verbs expressing a reversal of some actionor state, or removal, deprivation, release, etc. (unbend; uncork; unfasten; etc.), or to intensify the force of a verb already having such a meaning (unloose)