Struck by shock, terror, or amazement.
[Middle English agast, past participle of agasten, to frighten : a-, intensive pref. (from Old English ā-) + gasten, to frighten (from Old English gǣstan, from gāst, ghost).]
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Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.