1610s, "lack of ability, powerlessness," from French incapacité (16c.), from Medieval Latin incapacitatem (nominative incapacitas), from Late Latin incapax (genitive incapacis) "incapable," from in- "not" (see in- (1)) + Latin capax "capable," literally "able to hold much," from capere "to take," from PIE root *kap- "to grasp." As a legal term (1640s), "lack of qualification," referring to inability to take, receive, or deal with in some way.
word-forming element used in forming nouns from Latin words ending in -atus, -atum (such as estate, primate, senate). Those that came to English via French often arrived with -at, but an -e was added after c. 1400 to indicate the long vowel. The suffix also can mark adjectives formed from Latin past participles in -atus, -ata (such as desolate, moderate, separate); again, they often were adopted in Middle English as -at, with an -e appended after c. 1400.