The proverb advises against assessing the value of something you have been given for free. If someone gives you a gift, you should accept it graciously rather than evaluating whether it meets your standards. Looking a gift horse in the mouth implies ingratitude — you are more interested in what the gift is worth than in the generosity behind it.
A horse's teeth grow throughout its life and recede at the gum line with age. An experienced buyer could estimate a horse's age — and therefore its value — by examining its teeth. Short, sharp teeth indicated youth and fitness; long, worn teeth indicated old age and diminishing usefulness. When you were given a horse as a gift, inspecting its teeth to assess its market value was considered deeply ungrateful.
The concept appears as a Latin proverb in the 4th or 5th century AD, attributed to St Jerome: 'noli equi dentes inspicere donati' ('do not inspect the teeth of a given horse'). In English, John Heywood's 1546 proverb collection contains the form 'No man ought to looke a geuen hors in the mouth.' The proverb has equivalents in almost every European language, suggesting it originated independently from universal experience with horses.
Because it was the act of a buyer checking value before purchase. Doing this with a gift implied you were more concerned with its monetary worth than with the generosity of the giver — a social slight in any culture that values gift-giving.
Completely. The phrase applies to any gift, favour, or opportunity that comes without cost. Complaining about a free holiday because the hotel is not five-star, or criticising a handed-down piece of furniture, are modern equivalents.