A knight in shining armour
A person, usually a man, who comes to the aid of another, usually a woman, in a gallant and courteous manner.
The present-day use of this phrase is, of course, figurative and refers back to the notion of gallant knights saving fair maidens in distress. The reality behind that imagery is dubious and it no doubt owes much to the work of those Victorian novelists and painters who were captivated by the chivalrous ideal of an imagined court of Camelot. Nevertheless, knights did wear armour and that worn by royalty and the high nobility was highly polished and did in fact gleam and shine.
The earliest reference that I've found to the phrase in print dates from the late 18th century - in The British journal The Monthly Review, 1790, in a poem called Amusement: A Poetical Essay, by Henry Pye:
No more the knight, in shining armour dress'd
Opposes to the pointed lance his breast
Many of the 19th century citations describe imaginary knights who ride to the rescue of swooning maidens. That's almost, but not quite, the figurative use we have now - present day 'knights in shining armour' may dress as they please. The earliest uses I've found that summon up the 'shining armour' image in other contexts come from the USA (it's 'armor' there of course); for example, this piece from The Kenosha Times, September 1857:
"The ticket nominated is composed of able, earnest, honest men - of men by their reputation for personal worth and integrity protected from assaults as by a shining armor."