Spoil can mean to ruin or damage something, or to treat someone too kindly and make them spoiled.
When we spoil something, like a relationship by being unfair or a gift by not taking care of it, that usually means we messed things up. But when we say "spoil the fun" or "spoil a surprise", it's more about making an experience less enjoyable than expected.
How common is "spoil"?
Word spoil is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
(Also in plural: spoils) Plunder taken from an enemy or victim.
Material (such as rock or earth) removed in the course of an excavation, or in mining or dredging. Tailings.
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verb
To strip (someone who has been killed or defeated) of their arms or armour.
To strip or deprive (someone) of their possessions; to rob, despoil.
To plunder, pillage (a city, country etc.).
To carry off (goods) by force; to steal.
To ruin; to damage (something) in some way making it unfit for use.
To ruin the character of, by overindulgence; to coddle or pamper to excess.
Of food, to become bad, sour or rancid; to decay.
Example: Make sure you put the milk back in the fridge, otherwise it will spoil.
To render (a ballot paper) invalid by deliberately defacing it.
To reveal the ending or major events of (a story etc.); to ruin (a surprise) by exposing it ahead of time.