Rectangle
late 16th century: from medieval Latin rectangulum, from late Latin rectiangulum, based on Latin rectus ‘straight’ + angulus ‘an angle’.
wiktionary
Borrowed from Middle French, from Old French, from Medieval Latin or Late Latin rectangulum(“right angle”), from Latin rectus(“right”) + angulus(“an angle”).
etymonline
rectangle (n.)
in geometry, "quadrilateral plane figure having all its angles right and all its opposite sides equal," 1570s, from French rectangle (16c.), from rect-, combining form of Latin rectus "right" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line") + Old French angle (see angle (n.)). Late/Medieval Latin rectiangulum meant "a triangle having a right angle," noun use of neuter of rectiangulus "having a right angle." When the adjacent sides are equal, it is a square, but rectangle usually is limited to figures where adjacent sides are unequal.
