late 14c., from Old French intestat (13c.) and directly from Latin intestatus "having made no will," from in- "not" (see in- (1)) + testatus, past participle of testari "make a will, bear witness" (see testament). As a noun, "one who has not made out a will," from 1650s.
word-forming element making nouns of quality, state, or condition, a confusion in English of three similar suffixes from Latin: 1. in primacy, etc., from Old French -acie and directly from Medieval Latin -acia, Late Latin -atia, making nouns of quality, state, or condition from nouns in -as. 2. in advocacy, etc., from Late Latin -atia, forming nouns of state from nouns in -atus. 3. in fallacy, etc., from Latin -acia, forming nouns of quality from adjectives in -ax (genitive -acis). Also forming part of -cracy. Extended in English to nouns not found in Latin (accuracy) and to non-Latin words (piracy).
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Definitions of intestacy from WordNet
the situation of being or dying without a legally valid will;