late 14c., "a proposition inadvertently proved in proving another," from Late Latin corollarium "a deduction, consequence," from Latin corollarium, originally "money paid for a garland," hence "gift, gratuity, something extra;" and in logic, "a proposition proved from another that has been proved." From corolla "small garland," diminutive of corona "a crown" (see crown (n.)).
Also in Middle English "a follower, a sycophant" (late 14c.). As an adjective, "of the nature of a corollary," mid-15c.