'Rectangle' fuses PIE *h3reg- (straight) and *h2enk- (to bend) — straight angles, etymologically.
A plane figure with four straight sides and four right angles, especially one with unequal adjacent sides.
From Late Latin "rectangulum" (a right angle, and by extension a figure with right angles), a compound of "rectus" (right, straight, correct) and "angulus" (angle, corner). The first element "rectus" is the past participle of "regere" (to rule, guide, keep straight), from PIE *h₃reǵ- (to straighten, direct, rule), an enormously productive root that gave rise to Latin "rēx" (king), "rēgula" (rule), Sanskrit "rā́jan" (king), Old Irish "rí" (king), Gothic "reiks" (ruler), and English "right," "regal," "regulate," and "regime." The second element "angulus" derives from PIE *h₂enk- (to bend), which also produced
A 'rectangle' is literally a 'right angle.' But here is the paradox: 'right' comes from PIE *h₃reǵ- (straight), while 'angle' comes from PIE *h₂enk- (to bend). So a rectangle is a 'straight bend' — a contradiction in terms at the PIE level, naming a shape where straightness and bending meet at 90 degrees.