Meaning

Originally meaning the central aperture of the eye. Figuratively it is something, or more usually someone, cherished above others.

Origin

The phrase is exceedingly old and first appears in Old English in a work attributed to King Aelfred (the Great) of Wessex, AD 885, entitled Gregory’s Pastoral Care. The earliest recorded use in modern English is in Sir Walter Scott’s Old Mortality, 1816:

“Poor Richard was to me as an eldest son, the apple of my eye.”

Meaning

Those of great worth and reliability.

Origin

From the Bible, Matthew 5:13 (King James Version):

“Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.”

- Special thanks to Gary Martin of The Phrase Finder

Meaning

One who is dominant or victorious.

Origin

‘Top dog’ is synonymous with the similar, if now archaic, phrases, ‘upper dog’ and ‘over dog’. It is also the antonym of ‘under dog’ (now usually spelled as a single word) and its archaic synonym, ‘bottom dog’.
We have here a golden opportunity for those who consider plausibility to be enough evidence to hang a phrase derivation on and the following explanation has been widely repeated. When wooden planks were sawn by hand, two men did the job using a two-handed saw. The senior man took the top handle, standing on the wood, and the junior took the bottom, in the saw-pit below. Add to this the fact that the irons that were used to hold the wood were called dogs and that the bottom position was much the more uncomfortable, and we can jump from this scene to the origin of ‘top dog’ and ‘underdog’.

Meaning

Begin (again) from the beginning, embark on something without any preparation or advantage.

Origin

‘Start from scratch’ is an expression which has altered slightly in meaning since it was first coined. It is now usually used to mean ’start again from the beginning’ – where an initial attempt has failed and a new attempt is made with nothing of value carried forward from the first attempt (as opposed to ‘made from scratch’ which means ‘made from basic ingredients’).