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mid-15c., "capable of being considered," from Medieval Latin considerabilis "worthy to be considered," from Latin considerare (see consider). Meaning "pretty large" is from 1640s (implied in considerably).
CONSIDERABLE. This word is still frequently used in the manner out by Dr. Witherspoon in the following remark: "He is considerable of a surveyor; considerable of it may found in the country. This manner of speaking in the northern parts." [Pickering, 1816]