1. Don't look a gift horse in the mouth.
Meaning: Don't be ungrateful when you receive a gift.
Origin: This comes into the category of phrases called proverbs, that is, 'short and expressive sayings, in common use, which are recognized as conveying some accepted truth or useful advice'.
As horses age their teeth begin to project further forward each year and so their age can be estimated by checking how prominent the teeth are. This incidentally is also the source of another teeth/age related phrase - long in the tooth.
The advice given in the 'don't look...' proverb is: when given a present, be grateful for your good fortune and don't look for more by examining it to assess its value.
As with most proverbs the origin is ancient and unknown. We have some clues with this one however. The phrase was originally "don't look a given horse in the mouth" and first appears in print in 1546 in John Heywood's A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue, where he gives it as:
"No man ought to looke a geuen hors in the mouth."
Heywood is an interesting character in the development of English. He was employed at the courts of Henry VIII and Mary I as a singer, musician, and playwright. His Proverbs is a comprehensive collection of those known at the time and includes many that are still with us:
- Many hands make light work.
- Rome wasn't built in a day.
- A good beginning makes a good ending.
2. Straight from the Horse's Mouth
Meaning: From the highest authority.
Origin: In horse racing circles tips on which horse is a likely winner circulate amongst punters. The most trusted authorities are considered to be those in closest touch with the recent form of the horse, i.e. stable lads, trainers etc. The notional 'from the horse's mouth' is supposed to indicate one step better than even that inner circle, i.e. the horse itself.
It is a 20th century phrase. The earliest printed version is from the USA and clearly indicates the horseracing context - in the Syracuse Herald, May 1913.
"I got a tip yesterday, and if it wasn't straight from the horse's mouth it was jolly well the next thing to it."
3. Win Hands Down
Meaning: Win easily, with little effort.
Origin: Many phrases have originated as jargon in the world of sport, including several from boxing. The memory of Muhammad Ali dancing round the ring with his hands at his side and going on the win the bout might encourage us to think that that is the origin of this one. Not quite, although it does come from sport - horse racing.
Jockeys need to keep a tight rein in order to encourage their horse to run. Anyone who is so far ahead that he can afford to slacken off and still win he can drop his hands and loosen the reins - hence winning 'hands down'.
This is recorded from the mid 19th century. For example, 'Pips' Lyrics & Lays, 1867:
"There were good horses in those days, as he can well recall, But Barker upon Elepoo, hands down, shot by them all."
It began to be used in a figurative sense, to denote an easy win in other contexts, from the early 20th century.
Source: The Phrase Finder