to release the shift key, as on a typewriter or the keyboard of a computer terminal
Word origin
[1965–70; un-2 + shift]This word is first recorded in the period 1965–70. Other words that entered Englishat around the same time include: T cell, double-book, isometric exercise, overdub, red-eyeun- 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)