Damage, Loss or Harm

mid-15c., "legal claim or right; a concern; a benefit, advantage, a being concerned or affected (advantageously)," from Old French interest "damage, loss, harm" (Modern French intérêt), from noun use of Latin interest "it is of importance, it makes a difference," third person singular present of interresse "to concern, make a difference, be of importance," literally "to be between," from inter "between" (see inter-) + esse "to be" (from PIE root *es- "to be"). The sense development to "profit, advantage" in French and English is not entirely clear.