What does 含饴弄孙 mean? What does 含饴弄孙 mean?

What's its origin, what sort of contexts is it usually used in and also how popular an idiom is it?

It does not seem to appear in any regular Chinese-English dictionary I've found, and the Chinese idiom guides seem pretty cryptic about this (to me anyway).
It is a popular idiom that describes the leisure life of elderly people.

含: keep something in mouth
饴: syrup / candy / suger
弄: play with
孙: grandson

So this idiom means to play with grandchildren with candy in mouth.

You can also check its Chinese explanations at 含饴弄孙:

hán yí nòng sūn

含饴弄孙

【近义】天伦之乐
【释义】含着糖逗小孙子玩。形容晚年生活的乐趣。
【出处】《后汉书·明德马皇后纪》：“吾但当含饴弄孙，不能复知政事。”
【用例】佳儿佳妇，吾将～，不复理家政矣。（清·淮阴百一居士《壶天录》）

The links in my answer point to an online Chinese English dictionary, which has usage examples that extracted from China Central Television (CCTV) news. You can see the most fresh examples of the words you are looking for.