Horizontal

google

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mid 16th century (in horizontal (sense 3 of the adjective)): from French, or from modern Latin horizontalis, from late Latin horizon, horizont- (see horizon).

wiktionary

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Borrowed from French horizontal.

etymonline

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horizontal (adj.)

1550s, "relating to or near the horizon," from French horizontal, from Latin horizontem (see horizon). Meaning "flat" (i.e., "parallel to the horizon") is from 1630s. As a noun also from 1550s. Related: horizontally.