mid-15c., "capable of being considered, conceivable," from Medieval Latin considerabilis "worthy to be considered," from Latin considerare "to look at closely, observe," probably literally "to observe the stars," from assimilated form of com "with, together" (see con-) + sidus (genitive sideris) "heavenly body, star, constellation" (see sidereal).
Meaning "pretty large" is from 1650s (implied in considerably), from now-archaic earlier sense of "Worthy of regard or attention" (1610s).
CONSIDERABLE. This word is still frequently used in the manner pointed out by Dr. Witherspoon in the following remark: "He is considerable of a surveyor; considerable of it may be found in the country. This manner of speaking prevails in the northern parts." [Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," 1816]
1590s, "incalculable;" from 1630s as "not worthy of consideration or notice," from French inconsidérable (16c.), from in- "not" (see in- (1)) + considérable (see considerable). Related: Inconsiderably. OED has found an instance of the rare verb inconsider from 1697.
"to a considerable extent," expressing a degree less than very, 1560s, from pretty (adj.). Pretty much "in a considerable degree" is by 1660s.
a Scottish variant of gay (compare gray/grey), used 18c.-19c. also with the Scottish sense of "considerable, pretty much, considerably."
nightmare or demon that causes nightmares, c. 1600, from Greek Ephialtes, name of a demon supposed to cause nightmares; the ancient explanation is that it was from ephallesthai "to leap upon," which suits the sense, but OED finds "considerable" phonological difficulties with this.