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Webster 1913 Edition

Verb.

[1st pref.

un-

seat

To throw from one’s seat; to deprive of a seat.

Cowper.

Specifically, to deprive of the right to sit in a legislative body, as for fraud in election.

Macaulay.

Verb.

To throw from the seat.

unseat ‎(third-person singular simple present unseats, present participle unseating, simple past and past participle unseated)

To throw from one's seat; to deprive of a seat.
Specifically, to deprive of the right to sit in a legislative body, as for fraud in election, or simply by defeating them in an election.