late 14c., "steadfast, resolute; patient, unshakable; fixed or firm in mind," from Old French constant (14c.) or directly from Latin constantem (nominative constans) "standing firm, stable, steadfast, faithful," present participle of constare"to stand together," from assimilated form of com "with, together" (see con-) + stare "to stand," from PIE root *sta- "to stand, make or be firm."
Meaning "steadfast in attachment to a person or cause" is from early 15c. Of actions and conditions, "fixed, not varying" (1540s); "continual, enduring" (1650s). Meaning "regularly recurring" is from 1817. Related: Constantly.