Tricky Words in this week's OVI

Senior citizens. I think this is an example of PC ("politically-correct") vocabulary which has been pretty well accepted into ordinary English usage. This may be partly due to its being quite easy to say, because of the /s - s/ alliteration in the pronunciation of the words. It may also be partly because we unconsciously feel we should pay more respect to older people, as if there's not enough respect in words like "the elderly" (postarší), "pensioners" or "retirees" (dôchodcovia).
Showing more respect is different from avoiding offense, which seems to be the intention behind some other PC expressions, especially ones including the word "challenged" (literally "vyzvaný", but in fact meaning "postihnutý"). I suspect some of these of being jokes, like "vertically-challenged" for short people, "gravitationally-challenged" for overweight people, and "follicularly-challenged" for bald people.

Andy's Wordshop

Expressions based on horses. "From the horse's mouth" means to get first-hand (probably true) information, because a horse's teeth are the clearest indicator of its age (which by the way is also the origin of "long in the tooth", meaning old, because the gums - ďasná - recede with increasing age). "Don't look a gift horse in the mouth" means you shouldn't ask too many questions or be doubtful or suspicious (podozrievavý) when somebody wants to give you something - "gift" means "darovaný" here, and again, you look in a horse's mouth to check its age, to make sure the giver is telling the truth. "It's no use locking the stable door when the horse has bolted" means it's not worth being careful when it's already too late (the horse has run away).