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Webster 1913 Edition

Adj.

With gaunt or slender legs. (?)

“An armgaunt steed.”

Shak.

☞ This word is peculiar to Shakespeare. Its meaning has not yet been satisfactorily explained.

Adj.

slender, as the arm. [Not in use.]

armgaunt ‎(comparative more armgaunt, superlative most armgaunt)

(nonce word, obsolete) Of uncertain meaning; perhaps describing gaunt or slender legs.
An armgaunt steed. — Shakespeare.